Thank you Ric.
It happenns often especially while running when more ram and cpu usage is required. For instance, running a swf in browser may cause this to happen OR compiling a CPU intensive application. Sometimes, it just happens without any reason and the computer freezes, and other times a grey screen will come with a prompt to restart.
I have a G5 with dual 1.8 ghz IBM processors
OS 10.4 with the latest updates
Well my Mac started crumbling 3 months after I purchased it. It's been exactly two years. I took it to a repair shop and was told that the CPUs were replaced under Apple's warranty, it continued to happen. I took to another shop and they told me that the logic board has been replaced under Apple's warranty. The problem exists to this day.
-Abdullah