IMac boot issues, panic report since Yosemite upgrade

Question

I've been having serious issues getting my iMac to boot ever since upgrading to Yosemite - Basically it turns on, makes the chiming sound, goes to the white screen with the apple logo and progress bar and at about 1/3 of the way, goes blank white/black, then either reboots on a loop or freezes until I'm forced to manually shut it down.
When I boot in safe mode, I get this ProblemReport:
Anonymous UUID:       CFB3276F-B26B-291C-F495-653576C46E49
Wed Oct 22 16:58:32 2014
*** Panic Report ***
panic(cpu 0 caller 0xffffff800c77a590): "a freed zone element has been modified in zone ipc importance inherit: expected 0x58a9e1599e00910 but found 0xbfbfbf00bfbfbf, bits changed 0x53521aa995fb6af, at offset 0 of 88 in element 0xffffff801f2c7e40, cookies 0x3f001114074f583a 0x53521aa995fb6af"@/SourceCache/xnu/xnu-2782.1.97/osfmk/kern/zalloc.c:496
Backtrace (CPU 0), Frame : Return Address
0xffffff811662b910 : 0xffffff800c73a811
0xffffff811662b990 : 0xffffff800c77a590
0xffffff811662ba00 : 0xffffff800c779c17
0xffffff811662ba40 : 0xffffff800c778de7
0xffffff811662bb70 : 0xffffff800c720a3b
0xffffff811662bc40 : 0xffffff800c7337ff
0xffffff811662bcb0 : 0xffffff800c72b873
0xffffff811662bd00 : 0xffffff800cbb89e0
0xffffff811662bdc0 : 0xffffff800cbb6fc0
0xffffff811662be30 : 0xffffff800cbb8860
0xffffff811662bf20 : 0xffffff800cbb6627
0xffffff811662bf50 : 0xffffff800cc4dcb2
0xffffff811662bfb0 : 0xffffff800c83ac46
BSD process name corresponding to current thread: diagnosticd
Mac OS version:
14A389
Kernel version:
Darwin Kernel Version 14.0.0: Fri Sep 19 00:26:44 PDT 2014; root:xnu-2782.1.97~2/RELEASE_X86_64
Kernel UUID: 89E10306-BC78-3A3B-955C-7C4922577E61
Kernel slide:     0x000000000c400000
Kernel text base: 0xffffff800c600000
__HIB  text base: 0xffffff800c500000
System model name: iMac11,1 (Mac-F2268DAE)
System uptime in nanoseconds: 24152197674
last loaded kext at 23160986348: com.apple.driver.AppleBluetoothMultitouch 85.3 (addr 0xffffff7f8e5df000, size 61440)
loaded kexts:
com.apple.driver.AppleBluetoothMultitouch 85.3
com.apple.driver.AppleHDAHardwareConfigDriver 266.5
com.apple.driver.AppleTyMCEDriver 1.0.2d2
com.apple.driver.AGPM 100.14.37
com.apple.driver.AppleMikeyHIDDriver 124
com.apple.driver.AppleHDA 266.5
com.apple.iokit.IOBluetoothSerialManager 4.3.0f10
com.apple.filesystems.autofs 3.0
com.apple.driver.AppleOSXWatchdog 1
com.apple.driver.AppleMikeyDriver 266.5
com.apple.driver.AppleHWSensor 1.9.5d0
com.apple.driver.AppleUpstreamUserClient 3.6.1
com.apple.kext.AMDFramebuffer 1.2.8
com.apple.iokit.IOUserEthernet 1.0.1
com.apple.iokit.IOBluetoothUSBDFU 4.3.0f10
com.apple.ATIRadeonX2000 10.0.0
com.apple.iokit.BroadcomBluetoothHostControllerUSBTransport 4.3.0f10
com.apple.driver.AppleLPC 1.7.3
com.apple.driver.ACPI_SMC_PlatformPlugin 1.0.0
com.apple.kext.AMD4800Controller 1.2.8
com.apple.Dont_Steal_Mac_OS_X 7.0.0
com.apple.driver.AppleHWAccess 1
com.apple.driver.AppleHV 1
com.apple.driver.AppleMuxControl 3.7.21
com.apple.driver.AppleBacklight 170.4.12
com.apple.driver.AppleMCCSControl 1.2.10
com.apple.driver.AppleIRController 327.5
com.apple.driver.AppleUSBCardReader 3.5.0
com.apple.driver.AppleFileSystemDriver 3.0.1
com.apple.AppleFSCompression.AppleFSCompressionTypeDataless 1.0.0d1
com.apple.AppleFSCompression.AppleFSCompressionTypeZlib 1.0.0d1
com.apple.BootCache 35
com.apple.iokit.SCSITaskUserClient 3.7.0
com.apple.driver.XsanFilter 404
com.apple.iokit.IOAHCIBlockStorage 2.6.5
com.apple.driver.AppleUSBHub 705.4.1
com.apple.driver.AppleFWOHCI 5.5.2
com.apple.driver.AirPort.Atheros40 700.74.5
com.apple.iokit.AppleBCM5701Ethernet 10.1.2b3
com.apple.driver.AppleAHCIPort 3.0.7
com.apple.driver.AppleUSBEHCI 705.4.14
com.apple.driver.AppleUSBUHCI 656.4.1
com.apple.driver.AppleRTC 2.0
com.apple.driver.AppleACPIButtons 3.1
com.apple.driver.AppleHPET 1.8
com.apple.driver.AppleSMBIOS 2.1
com.apple.driver.AppleACPIEC 3.1
com.apple.driver.AppleAPIC 1.7
com.apple.driver.AppleIntelCPUPowerManagementClient 218.0.0
com.apple.nke.applicationfirewall 161
com.apple.security.quarantine 3
com.apple.security.TMSafetyNet 8
com.apple.driver.AppleIntelCPUPowerManagement 218.0.0
com.apple.driver.IOBluetoothHIDDriver 4.3.0f10
com.apple.driver.AppleMultitouchDriver 260.30
com.apple.driver.DspFuncLib 266.5
com.apple.kext.OSvKernDSPLib 1.15
com.apple.iokit.IOAudioFamily 200.6
com.apple.vecLib.kext 1.2.0
com.apple.iokit.IOSerialFamily 11
com.apple.kext.triggers 1.0
com.apple.iokit.IOSurface 97
com.apple.iokit.IOFireWireIP 2.2.6
com.apple.iokit.IOBluetoothHostControllerUSBTransport 4.3.0f10
com.apple.iokit.IOUSBUserClient 705.4.0
com.apple.driver.AppleHDAController 266.5
com.apple.iokit.IOHDAFamily 266.5
com.apple.driver.AppleSMBusPCI 1.0.12d1
com.apple.driver.IOPlatformPluginLegacy 1.0.0
com.apple.driver.IOPlatformPluginFamily 5.8.0d49
com.apple.kext.AMDSupport 1.2.8
com.apple.AppleGraphicsDeviceControl 3.7.21
com.apple.iokit.IOBluetoothFamily 4.3.0f10
com.apple.driver.AppleSMC 3.1.9
com.apple.driver.AppleGraphicsControl 3.7.21
com.apple.driver.AppleBacklightExpert 1.1.0
com.apple.iokit.IONDRVSupport 2.4.1
com.apple.driver.AppleSMBusController 1.0.13d1
com.apple.iokit.IOGraphicsFamily 2.4.1
com.apple.iokit.IOUSBHIDDriver 705.4.0
com.apple.iokit.IOSCSIBlockCommandsDevice 3.7.0
com.apple.iokit.IOUSBMassStorageClass 3.7.0
com.apple.driver.AppleUSBMergeNub 705.4.0
com.apple.driver.AppleUSBComposite 705.4.9
com.apple.iokit.IOSCSIMultimediaCommandsDevice 3.7.0
com.apple.iokit.IOBDStorageFamily 1.7
com.apple.iokit.IODVDStorageFamily 1.7.1
com.apple.iokit.IOCDStorageFamily 1.7.1
com.apple.iokit.IOAHCISerialATAPI 2.6.1
com.apple.iokit.IOSCSIArchitectureModelFamily 3.7.0
com.apple.iokit.IOFireWireFamily 4.5.6
com.apple.iokit.IO80211Family 700.52
com.apple.iokit.IOEthernetAVBController 1.0.3b3
com.apple.driver.mDNSOffloadUserClient 1.0.1b8
com.apple.iokit.IONetworkingFamily 3.2
com.apple.iokit.IOAHCIFamily 2.7.0
com.apple.iokit.IOUSBFamily 705.4.14
com.apple.driver.AppleEFINVRAM 2.0
com.apple.driver.AppleEFIRuntime 2.0
com.apple.iokit.IOHIDFamily 2.0.0
com.apple.iokit.IOSMBusFamily 1.1
com.apple.security.sandbox 300.0
com.apple.kext.AppleMatch 1.0.0d1
com.apple.driver.AppleKeyStore 2
com.apple.driver.AppleMobileFileIntegrity 1.0.5
com.apple.driver.AppleCredentialManager 1.0
com.apple.driver.DiskImages 389.1
com.apple.iokit.IOStorageFamily 2.0
com.apple.iokit.IOReportFamily 31
com.apple.driver.AppleFDEKeyStore 28.30
com.apple.driver.AppleACPIPlatform 3.1
com.apple.iokit.IOPCIFamily 2.9
com.apple.iokit.IOACPIFamily 1.4
com.apple.kec.corecrypto 1.0
com.apple.kec.Libm 1
com.apple.kec.pthread 1
Model: iMac11,1, BootROM IM111.0034.B02, 4 processors, Intel Core i7, 2.8 GHz, 8 GB, SMC 1.54f36
Graphics: ATI Radeon HD 4850, ATI Radeon HD 4850, PCIe, 512 MB
Memory Module: BANK 0/DIMM0, 2 GB, DDR3, 1067 MHz, 0x80CE, 0x4D34373142353637334548312D4346382020
Memory Module: BANK 1/DIMM0, 2 GB, DDR3, 1067 MHz, 0x80CE, 0x4D34373142353637334648302D4346382020
Memory Module: BANK 0/DIMM1, 2 GB, DDR3, 1067 MHz, 0x80CE, 0x4D34373142353637334648302D4346382020
Memory Module: BANK 1/DIMM1, 2 GB, DDR3, 1067 MHz, 0x80CE, 0x4D34373142353637334548312D4346382020
AirPort: spairport_wireless_card_type_airport_extreme (0x168C, 0x8F), Atheros 9280: 4.0.74.0-P2P
Bluetooth: Version 4.3.0f10 14890, 3 services, 27 devices, 0 incoming serial ports
Network Service: Wi-Fi, AirPort, en1
Serial ATA Device: Hitachi HDS722020ALA330, 2 TB
Serial ATA Device: OPTIARC DVD RW AD-5680H
USB Device: Hub
USB Device: Internal Memory Card Reader
USB Device: BRCM2046 Hub
USB Device: Bluetooth USB Host Controller
USB Device: Hub
USB Device: IR Receiver
USB Device: Built-in iSight
Thunderbolt Bus:
I have a late 2009, 27" iMac, 2.8 GHz, 8gb Ram - was running Mavericks, upgraded to Yosemite
Ive run disk utility and verified disk multiple times - on one occasion it said the HD was corrupt but successfully fixed and hasn't reappeared since.
I've also run the hardware test (hold down D as computer restarts) with no problems found
And the ram reset thing (hold down R when computer restarts)
Ive also reinstalled Yosemite and clean Installed it so my computer is now empty.
I've also taken out each piece of ram and tried booting with each individual piece (the same problem occurred with each piece)

Answer 1

I am experiencing this same exact problem; however, I upgraded to Yosemite a couple of weeks ago and didn't experience any problems until this past Friday.  The only other thing that's been done since then was installing some updated drivers for my daughter's Wacom Intuos Tablet.  Everything was fine for a bit and then (as best we can tell), the video froze up on my son while he was using it and he walked away without shutting it down.  We *think* it got overheated and appears to have resulted in the ATI Radeon HD 4850 now causing kernel panics during boot-up.
Symptoms:
On boot, the chime plays and apple logo appears and the progress bar begins to fill in until it gets about 1/4 of the way across.
After the progress bar gets about 1/4 of the way across, the screen goes gray/white, then black while the machine shuts down and re-starts
The machine starts back up and there is an error message (sometimes) that appears and says something went wrong during boot.
After a bit (or if I press a key), the whole cycle starts all over again with Step 1. above.
I've done the following, myself, to no avail:
Reset the SMC (unplug the power and all cables, wait 15-20 seconds and plug the power back in) - didn't help
Reset the NVRAM/PRAM - didn't help
Attempted to boot into Safe Mode - boots and then the display shifts to the left and has garbled/pixelated squares on the bottom right corner
Attempted to boot from the 10.6.2 DVD that came with the iMac when we purchased it back in November, 2011 - boots and then the entire display is pixelated/garbled
Ran the Apple Hardware Test (simple (5+ minutes) and extensive (1+ hours) - no problems found by either, but it didn't appear that the GPU was tested during this diagnostic
Verified that the EFI and SMC firmware is up to date with the latest for the iMac 11,1
Booted into the Recovery Partition (this is the only thing that has succeeded)
Recovery Mode:  Ran the Disk Utilities to verify the 1TB HD - no issues with the disk or with permissions.
Recovery Mode:  Created a .dmg of my entire 1TB Disk to a backup drive (No time machine backups...I know, bad practice on my part).
Recovery Mode:  Attempted to re-install Mac OS X Yosemite - problem persists on reboot
Recovery Mode:  Erased and Low-Level (1 pass) write of the entire 1TB partition on the HD and re-installed Mac OS X Yosemite - problem persists on reboot
When I attempt to boot into Safe Mode (Verbose: Command + Shift + V rather than Command + Shift), the messages scroll from the screen before I can catch what it's saying, but it appears to be stating that the machine is re-booting after a kernel panic due to display problems...which correlates with the garbled/pixelated display.
I've found a couple of other posts on here and they seem to point to the ATI Radeon HD 4850 and issues with it overheating and going tango uniform as well.
Basic Troubleshooting of Gray Screen Situations:  Mac OS X: Gray screen appears during startup - Apple Support
Specific Troubleshooting and ATI Radeon Info:  09 27" iMac 2.66 grey garbled screen, no boot up   10.8.3. /  Senior mods please
I've scheduled an appointment to take it into the Apple Store next Saturday (my first available) to have them verify my findings.  I'm hoping and praying that it doesn't cost an arm and a leg to repair/replace the GPU...can't really afford it right now.
At any rate, I hope this information is useful to you!