Sudden Shutdowns
It's not in sleep mode, it's off completely, as if the power had been pulled.
If I switch it on again, it comes on as usual. If I set the Power Saving settings so as to come back on again after power loss, it will automatically come back on after this unprompted shutdown.
I've reset the SMU, PRAM and NVRAM, verified and repaired permissions, done the full hardware test (which reported no problems), and it still happened again earlier today!
The only thing I've not tried is a verify disk and repair disk, as I need to make a backup first just in case and haven't had time lately. Then leaving it on overnight with the system disk in to see if it still shuts off.
In the logs, I see an AppleSMU message with shutdown cause -122 or -110.
It's not covered by the power supply support extension programme, as it's a 2nd generation G5 iMac (2Ghz, ALS), which has not had any hardware modifications.
It's going to be a pain to take it in to an Apple Store, so I'm trying to cover everything possible before resorting to that, especially as the fault is intermittent (didn't happen at all on Saturday) and may not happen at all if I take it in!
Thanks in advance for any further advice, and I'd like to hear if anyone else has had a similar problem and how they resolved it!
iMac G5 (2.0Ghz) Mac OS X (10.4.7)