IMac Freezing, tells me to power off

Question

Hi
My iMac PPC keeps freezing and telling me to power off or restart.
I notice the internal hard drive icon no longer appears. 
I had hoped to resolve this problem by installing my startup disc onto a Firewire drive, but it has not.  I can start from the drive and use the computer from anywhere of 5 to 60 minutes before it will freeze again.
Is there anything else I could self service?  Do these models have internal batteries that need to be changed?  I do keep the iMac plugged into a power strip that I turn off at night. 
It is always hard for me to know when to walk away from an older computer.

Answer 1

Thanks.  can you direct me to where I can find the battery model number.  I know it is somehwere on the Apple site, but I can't seem to find it.  I'd rather have the battery in hand before I open it up.
I am also pasting below the error message.  I have no idea how to interpret this.
Problem Report for Mac OS X Kernel
Interval Since Last Panic Report:  0 sec
Panics Since Last Report:          1
Anonymous UUID: FD1475F0-27DD-470A-9DD8-FF6EF42321CD
Wed Mar 14 08:53:21 2012
panic(cpu 0 caller 0x00044040): "zalloc: \"threads\" (329068 elements) retry fail 3"@/SourceCache/xnu/xnu-1228.15.4/osfmk/kern/zalloc.c:966
Latest stack backtrace for cpu 0:
      Backtrace:
         0x0009CC88 0x0009D63C 0x00029DA0 0x00044040 0x0003D034 0x0003D3F8 0x0003D490 0x00337D38
         0x00F32CA0 0x00F33E8C 0x00F33F70 0x00F29554 0x00F310F8 0x00F3357C 0x00F333EC 0x0003F2A0
         0x000B1DD4
      Kernel loadable modules in backtrace (with dependencies):
com.apple.iokit.IOATABlockStorage(2.0.6)@0xf26000->0xf38fff
            dependency: com.apple.iokit.IOATAFamily(2.0.1)@0x99f000
dependency: com.apple.iokit.IOStorageFamily(1.5.6)@0x7e9000
Proceeding back via exception chain:
   Exception state (sv=0x35ac1780)
      PC=0x00000000; MSR=0x0000D030; DAR=0x00000000; DSISR=0x00000000; LR=0x00000000; R1=0x00000000; XCP=0x00000000 (Unknown)
BSD process name corresponding to current thread: kernel_task
Mac OS version:
9L31a
Kernel version:
Darwin Kernel Version 9.8.0: Wed Jul 15 16:57:01 PDT 2009; root:xnu-1228.15.4~1/RELEASE_PPC
System model name: PowerMac8,1
System uptime in nanoseconds: 898565958496
vm objects:1481900
kernel map entries:2830300
kalloc.32:1368064
kalloc.64:2584576
vm pages:22374484
ipc ports:46470760
threads:308007648
uthreads:136895616
vnodes:1603440
HFS node:1569792
buf.8192:23126016
Kernel Stacks:1359872
Kalloc.Large:3514576
unloaded kexts:
com.apple.driver.Apple02DBDMAAudio         2.5.8f1 - last unloaded 5100439716
loaded kexts:
com.apple.filesystems.msdosfs  1.5.5 - last loaded 2933016418
com.apple.filesystems.autofs     2.0.2
com.apple.driver.AudioIPCDriver         1.0.6
com.apple.GeForcePPC           5.4.8
com.apple.driver.InternalModemSupport          2.4.0
com.apple.driver.MotorolaSM56K       1.3.9
com.apple.driver.AppleOnyxAudio       2.5.8f1
com.apple.driver.AppleTexasAudio      2.5.8f1
com.apple.driver.AppleTexas2Audio    2.5.8f1
com.apple.driver.AppleDACAAudio    2.5.8f1
com.apple.AppleOnboardDisplay         1.15.1
com.apple.driver.AppleSEP      1.3.0
com.apple.driver.PowerMac8_1_ThermalProfile           3.4.0d0
com.apple.driver.AppleSCCSerial        1.3.2
com.apple.driver.AppleK2Driver          1.7.2f1
com.apple.driver.AppleI2S       1.0.1f1
com.apple.driver.AppleLM7x   1.9d0
com.apple.iokit.IOFireWireIP   1.7.7
com.apple.driver.AppleVSP     2.2.2
com.apple.nvidia.nv30hal          5.4.8
com.apple.driver.AppleHWSensor       1.9d0
com.apple.driver.AppleSMUMonitor    1.9d0
com.apple.driver.ApplePulsar   1.5.2d0
com.apple.driver.AppleCPUThermo     2.0.0a0
com.apple.driver.AppleFileSystemDriver          1.1.0
com.apple.driver.AppleUSBHIDMouse            1.2.4b3
com.apple.driver.AppleUSBMergeNub            3.5.2
com.apple.iokit.IOUSBMassStorageClass        2.0.8
com.apple.driver.iTunesPhoneDriver     1.0
com.apple.driver.XsanFilter      2.7.91
com.apple.iokit.IOSCSIMultimediaCommandsDevice   2.1.1
com.apple.iokit.SCSITaskUserClient    2.1.1
com.apple.driver.LSI_FW_500            2.0.9
com.apple.driver.Oxford_Semi 2.0.9
com.apple.driver.initioFWBridge           2.0.9
com.apple.driver.IOFireWireSerialBusProtocolSansPhysicalUnit           2.0.9
com.apple.driver.StorageLynx  2.0.9
com.apple.iokit.IOATABlock
Model: PowerMac8,1, BootROM 5.2.2f4, 1 processor, PowerPC G5  (3.0), 1.8 GHz, 2 GB
Graphics: GeForce FX 5200, GeForce FX 5200, spdisplays_agp_device, 64 MB
Memory Module: DIMM0/J4000, 1 GB, DDR SDRAM, PC3200U-25440
Memory Module: DIMM1/J4001, 1 GB, DDR SDRAM, PC3200U-30330
Modem: Jump, V.92, Version 1.0
Network Service: Ethernet, Ethernet, en0
Serial ATA Device: WDC WD2500AAJS-41RYA0, 232.89 GB
Parallel ATA Device: MATSHITADVD-R   UJ-825
USB Device: SAC-Click free, (null) mA
USB Device: Hub in Apple Extended USB Keyboard, (null) mA
USB Device: Apple Optical USB Mouse, (null) mA
USB Device: Mass Storage, (null) mA
USB Device: Apple Extended USB Keyboard, (null) mA
FireWire Device: FreeAgent GoFlex, Seagate, 400mbit_speed