G5 Power failure during start up and wake

Question

I have an early G5 20" non-iSight iMac that has a power problem that is steadily getting worse.
As Apple support has expired and can't be extended, I'm at a loss how to resolve this.
Description.
The problem first started as intermittent during wake up from sleep (power light pulsing). I'd move the mouse or tap the keyboard to wake the iMac and I'd hear the disk just start to spin up followed by a sharp 'pop/click' like the power cord had been pulled, and it would be powered off needing the power button to be pressed to restart from scratch.
Now the problem has worsened in that occasionally I will find the iMac has completely powered down during sleep. When I restart it can take up to 6 or more attempts to get to the point where I can use it. There's a sequence of restarts needed, each starting from pushing the power button, and then ending during the starting sequence with a 'pop/click' as the power suddenly shuts off. Each time the start up will progress a little further than the previous attempt until eventually it finishes building the desk top after login and I can operate normally until the next time I find it powered off after a few hours of non-use. The power failure starts initially from just after the welcome chime, and progresses to maybe 5 or 6 times later to during the desk top build after log-in. However, once logged in and operating the iMac behaves as normal, switching users, log-out and login, screen save and wake from screen save etc are all as I'd expect. I've not (yet at least) had any sudden power failures during normal operation.
The serial number of the iMac falls within the Sept 05 Repair Extension Program serial number range (not EMC 2056 though), but when I spoke to Apple Support they said that the lack of random behaviour meant that the problem I'm having is not related and so would not be covered by this program.
So I'm kinda stuck with a problem that is annoying when it occurs, which is daily at the moment, although it's been this bad a few weeks back and then was fine (iMac left switched on for 2 to 3 weeks without a problem) until we had a power cut earlier this week, and it's started again.
I've searched through the forums and while I could find plenty of references to 'Sudden Power Loss' none seemed to be as predictable and similar to this one.
Suggestions and ideas welcome. Thanks.

Answer 1

i am having a similiar issue. I have the same exact computer (iMac G5, non-isight) and it stays on for 30 mins, hour, 3 hours and shuts off. Upon starting up, sometimes it doesn't make it to the login screen before shutting down again. I have downloaded heat temp monitor programs and it seems to get to about 70 degrees celcius sometimes. Upon restarting, I get the "report problem to apple" screen with this information. I have tried to restart the power supply too.
Thu Apr 17 09:02:18 2008
panic(cpu 0 caller 0x008E0BE4): "Missing CPU diode calibration information.\n"@/SourceCache/IOPlatformPluginFamily/IOPlatformPluginFamily-312. 0.1/SMUNeo2_PlatformPlugin/SMU_Neo2CPUTempSensor.cpp:124
Latest stack backtrace for cpu 0:
Backtrace:
0x0009B478 0x0009BE1C 0x00029DD4 0x008E0BE4 0x008D0308 0x008CFD78 0x008E309C 0x008D48E8
0x008E2C6C 0x00342B8C 0x00343CE0 0x00345B90 0x00344CB0 0x000B05D4
Kernel loadable modules in backtrace (with dependencies):
com.apple.driver.SMUNeo2PlatformPlugin(3.1.2d1)@0x8dc000->0x8eafff
dependency: com.apple.driver.IOPlatformPluginFamily(3.1.2d1)@0x8c9000
com.apple.driver.IOPlatformPluginFamily(3.1.2d1)@0x8c9000->0x8dbfff
Proceeding back via exception chain:
Exception state (sv=0x39e04c80)
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:
9C7010
Kernel version:
Darwin Kernel Version 9.2.2: Tue Mar 4 21:23:43 PST 2008; root:xnu-1228.4.31~1/RELEASE_PPC
System model name: PowerMac8,1
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