Assuming this isn't a general G5 related problem, and that your
iMac's hardware is healthy (ram, motherboard), the first thing I'd
do is disable all add-ons and try surfing the web for a
few hours to see if the problem is still there. You've got so many
add-ons doing so many different things simultaneously that it's
impossible to trouble-shoot without trying this first.
Chris: I have found the solution... erasing my profile (yep, the final solution). The problem was the AnonimoX addon on trying to auto-update. Weird thing, i ll be glad if you run some test on the add-on. Thanks for a quick response.
(ah, im not using an iMac, this one is a pro 2005)