It's as if there is a power cut, the computer just suddenly goes off. Then won't turn back on, unless you wait at least 20 minutes. Anyone hear of this before, and what is the reason, and how can I fix it????
It's happened when i've read an email, surfing the web, using Photoshop... no specific action seems to trigger it.
I had this problem with my 20" G5 iMac (with Auto Light Sensor).
The machine worked fine then it started to shut down at random moments in december 2008. It was very nerve racking: it would shut down right in the middle of a sentence I was reading, or at the end of the text I had written: I had to set auto-saves in MS Word at very short intervals... but you cannot do this withs all softwares. No screen problems, wake-up problems or performance deterioration.
I went to my local Apple Store Monday and discussed the problem with a Genius: I did not have time to finish my explanations, he replied "it's the power supply". I made a back-up (luckily the iMac did not quit during the process) and brought the machine in Tuesday morning. Since they had the part in stock, it was ready in the same day in the evening. Since then (48 hours), no unwanted power downs occured. Repair is garanteed 90 days.
I believe power supply problems are independent from motherboard problems. (The last time I had a similar problem was with my Mac 512 long time ago.) And this is not software related nor related to overheating (although keeping the inside tidy and dust free is never bad) so zapping the PRAM or PMU is, to my opinion, a loss of time for thi problem.
Note that this is valid for iMac G5 20" with ALS. I do not have experience with other types of problems on iMacs (bad screens, bad capacitors, etc).
If you are a good tinkerer, I do believe the part can be purchased and changed by yourself. However, I suggest that you first discuss the problem with a Apple Genius. Dont bother to bring your computer with you the first time, just discuss the problem and the cost of repair. Make sure they have the part in stock, then bring your iMac in for repair.
Your problem also sounds like the power supply problem , hahnphoto.
Your CPU usage doesn't sound particularly problematic. Printing, for example, will usually push CPU usage right up.
The trouble is increased CPU usage pushes up power consumption, which places heavier demands on the supply with the resulting shutdowns you mention.
Frequent power related crashes can certainly cause serious directory and data corruption. You should run disk utility from your OS DVD, or from an external drive after EVERY crash to "repair disk".
