I was working away as usual on my i-Mac G5 20" Rev.A today when it emitted a sort of muffled beep sound and shut down. After sitting blankly, staring at the screen in astonishment for 5 minutes, (never having had this happen before on any of my Macs), I tried re-starting. It went through the usual start-up process no problem but after a half a minute or so, the same thing happened. The only thing I could think of was that it might have overheated. Tried restarting once more to see if I could get a temperature reading but I didn't even get time to bring up dashboard before it shutdown. I reckoned maybe the air wasn't circulating properly, so I took it apart and cleaned the vents and got rid of the dust inside the case. It didn't really seem to be all that dirty. Everything seems to have been fine now for the last 8 hours and CPU and HD drive are both running at 53ºC. Am I good to go now or should I be worried?