there might be a prob
I have a rev A imac g5 20" at work ( I ordered it right after they were announced) and sometime last year, it developed wierd probs, the first symptom was a garbled screen. I would turn it on and the grey apple logo would loose it's definition and then I get to the login screen, it would look like every other line would shift 1 or 2 pixels to the right. when I would get into the system, some of my windows would corrupt. this only happened twice and I dismissed it, a reboot would fix it. finally 1 day, about 2 weeks later I would get kernal panics and the line shifting would happen constantly with no reboots fixing it. I called applecare and they told me that there was a prob with their capaciators popping and they would send a tech to switch out the mainboard. So, being a courious guy, I popped off the back cover to look and sure enough , a number of capaciators were bulging. so if I were you, I would make a backup of the data and call applecare, esp if it's a rev A imacg5.
on a side note, once the board was replaced, it's been steady ever since, no more probs