PPC G5 Power Mac and Overheating ATI video card

I am experiencing sudden shut down of my dual processor 1.8 gig Power Mac. The system simply shuts off. I have recently upgraded my install to 10.5.4. I put it on Tuesday evening the first of July. On the evening of the 2nd the system shut down on its own after the fans ramped up in speed. That was most disconcerting. I have the system on a good size Belkin UPS unit so I am dubious of attributing this problem to the lightning we had here on the day of the 2nd. Anybody with this experience? Please post. Thanks

Update:

Hardware failure is the root cause. I stressed the system using Tiger – 10.4.11. The fans gently ramp up and the heat out of the upper processor vent gets quite high. 

Seems my wifes computer fritzing the day before was no coincidence

Update 2:

I knocked a few degrees off the processors and the memory controller by blowing out the dust with compressed air. The system looks like it needs a thorough disassembly and cleaning by a pro. I wonder if the heatsinks could also benefit by renewing the heatsink compound as well.

 

Marcel Bresink Temperature Monitor

Update3

I traced the problem down to the video card running hot. I removed the fan from the board and disassembled it. I found a build-up of rubbery goop. Apparently the fan is being rev limited by the build up of the foreign material. So I cleaned it up with cotton tip swabs and S-100 Engine Brightener containing petroleum distillates and silicones. Then I lubricated the bearing well with Robbe No. 5530. After dabbing off the excess I reassembled the fan. I put the card back together and replaced it to the system and fired it up. Lo and behold the temperature of the indicator stayed steady and I haven’t crashed again. I stressed the system running Halo as usual and it runs properly once again. Yeehaa!