iMac G5
iMacG5
iMacG5
Hi! I fix these older units for a living and suspect that your power supply or your inverter board is on the way out. Good to do as has been suggested and back up data, but in the event that your machine dies before being able to do this there is very little danger of loosing data. In that event the Hard Drive can be removed and all data recovered to another Mac without much difficulty.
I would say your Mac is more like a 50 year-old than a 90 year-old :-) Having said that, it sounds like a 50 year-old in need of a good checkup and some replacement surgery . . .
iMac G5