PowerPC :: System Shutsdown Randomly / Fan Noise Is Supreloud

I have a problem... I use very processor heavy programs on my iMac G5 and up until about a year ago, I was getting along ok. Here's the gist of it:

1) World of Warcraft/ProTools etc. Prolonged use over many hours eventually caused my imac's fan noise to be unbearable. Even worse, my imac began randomly shutting down almost predictably during high processor use.

2) Was taken to an apple store (while in warranty) and only thing said was that it needed a new logic board. No explanation what actually happened to it. Logic board replaced. Few months later, it's happening again.

3) I am now outside an apple warranty and it will cost me $400+ to get it repaired. It is functional for regular internet use, but on sites like youtube, after a few videos it will likely shut down.

I have opened up my computer several times to inspect the capacitors, but they all look like new. I realize overuse is probably the problem, but if there is a fix I can do myself, I'd at least like to try. The problem is that I have no idea what to do I hate hearing "needs a new logic board" instead of "this part ____ needs to be replaced/fixed"

Specs: iMac G5 17" 2.0ghz generation right before isight macs.