Mac Pro Freeze constantly - no kernel panic, no crash reports. Any ideas?

Question

Dear all,
My good ol' Mac Pro (4.1-2009) which has been utterly reliable for years is now driving me cazy.
It freezes from time to time (let's say twice a day but sometimes not at all for 2 days in a row) - only possible option is hard reboot.
The screen just freezes, no kernel panic, no crash reports.
It has started after a ram replacement but then reverting to the original settings didn't change anything.
Tried all what possible: reverting to original ram (the Apple one), clean install on a new hard drive (10.8.4), replacing graphic card (4870 to 5770), Zap PRAM, resetting SMC, safe mode, clean new account, booting only with keyboard and mouse, Techtool Pro hardware tests-all passed (but not AHT as I can't find the original DVDs anymore), smc fan control, voodoo.
Some apps seem more enthusiastic at crashing than others especially Cinema 4D (when rendering), World of Warcraft (when playing), Photoshop CC (in OGL mode) but sometimes just dropping down a menu causes a freeze.
My eyes are full of tears.
Any idea? Someone had a similar experience?
I might start drinking.
Thanks.
Mac pro 4.1 - 2009 - 2x2.26 quad xeon.
OS X 10.8.4 on a brand new hard drive.
Ram 24 Gb (4/4/4/- 4/4/4/-)
ATI 5770

Answer 1

Got a freeze but this time I also got a report! For some reason it makes me happy.
as I said, starting with just the keyboard and the mouse is enough to make this ****' machine freeze.
Here we go:
Interval Since Last Panic Report:  122138 sec
Panics Since Last Report:          1
Anonymous UUID:
Mon Jul 22 23:31:57 2013
Machine-check capabilities 0x0000000000001c09:
family: 6 model: 26 stepping: 5 microcode: 17
Intel(R) Xeon(R) CPU           E5520  @ 2.27GHz
9 error-reporting banks
threshold-based error status present
extended corrected memory error handling present
Processor 0: no valid machine-check state
Processor 1: no valid machine-check state
Processor 2: no valid machine-check state
Processor 3: no valid machine-check state
Processor 4: no valid machine-check state
Processor 5: no valid machine-check state
Processor 6: no valid machine-check state
Processor 7: no valid machine-check state
Processor 8: machine-check status 0x0000000000000004:
machine-check in progress
MCA error-reporting registers:
IA32_MC0_STATUS(0x401): 0x0000000000000800 invalid
IA32_MC1_STATUS(0x405): 0xbe00000000400e0f valid
  MCA error code:            0x0e0f
  Model specific error code: 0x0040
Model: MacPro4,1, BootROM MP41.0081.B07, 8 processors, Quad-Core Intel Xeon, 2.26 GHz, 24 GB, SMC 1.39f5
Graphics: ATI Radeon HD 5770, ATI Radeon HD 5770, PCIe, 1024 MB
Memory Module: DIMM 1, 4 GB, DDR3 ECC, 1066 MHz, 0x855D, 0x000000000000000000000000000000000000
Memory Module: DIMM 2, 4 GB, DDR3 ECC, 1066 MHz, 0x838A, 0x5052494E4345544F4E000000000000000000
Memory Module: DIMM 3, 4 GB, DDR3 ECC, 1066 MHz, 0x855D, 0x000000000000000000000000000000000000
Memory Module: DIMM 5, 4 GB, DDR3 ECC, 1066 MHz, 0x066D, 0x5444344731384339452D5A38000000000000
Memory Module: DIMM 6, 4 GB, DDR3 ECC, 1066 MHz, 0x066D, 0x5444344731384339452D5A38000000000000
Memory Module: DIMM 7, 4 GB, DDR3 ECC, 1066 MHz, 0x066D, 0x5444344731384339452D5A38000000000000
Bluetooth: Version 4.1.4f2 12041, 2 service, 18 devices, 1 incoming serial ports
Network Service: Ethernet 1, Ethernet, en0
Network Service: Ethernet 2, Ethernet, en1
PCI Card: ATI Radeon HD 5770, sppci_displaycontroller, Slot-1
PCI Card: ATI Radeon HD 5770, ATY,HoolockParent, Slot-1
Serial ATA Device: HL-DT-ST DVD-RW GH41N
Serial ATA Device: SAMSUNG HD103UJ, 1 TB
Serial ATA Device: WDC WD5000AAKS-00A7B0, 500,11 GB
Serial ATA Device: WDC WD5000AAKS-00A7B0, 500,11 GB
Serial ATA Device: SAMSUNG HD501LJ, 500,11 GB
USB Device: USB 2.0 Drive, 0x152d  (JMicron Technology Corp.), 0x2329, 0xfa400000 / 3
USB Device: hub_device, apple_vendor_id, 0x911c, 0xfa200000 / 2
USB Device: HP LaserJet P1005, 0x03f0  (Hewlett Packard), 0x3d17, 0xfa210000 / 5
USB Device: Apple Cinema HD Display, apple_vendor_id, 0x921c, 0xfa220000 / 4
USB Device: hub_device, apple_vendor_id, 0x9130, 0xfd100000 / 3
USB Device: Keyboard Hub, apple_vendor_id, 0x1006, 0xfd110000 / 5
USB Device: G9x Laser Mouse, 0x046d  (Logitech Inc.), 0xc066, 0xfd113000 / 7
USB Device: Apple Keyboard, apple_vendor_id, 0x0221, 0xfd112000 / 6
USB Device: Apple Cinema Display, apple_vendor_id, 0x9222, 0xfd120000 / 4
USB Device: My Passport 0748, 0x1058  (Western Digital Technologies, Inc.), 0x0748, 0xfd500000 / 2
USB Device: BRCM2046 Hub, 0x0a5c  (Broadcom Corp.), 0x4500, 0x5a100000 / 2
USB Device: Bluetooth USB Host Controller, apple_vendor_id, 0x8215, 0x5a110000 / 3
FireWire Device: built-in_hub, 800mbit_speed
FireWire Device: LaCie d2 Extreme LUN 0, LaCie Group SA, 800mbit_speed