I just upgraded the video card in my G5 to the 6800 Ultra earlier today. Before installing it, I broke down the heatsink and fan, cleaned off the old thermal compound, and put on a fresh coat of Artic Silver MX5(taking all the normal precautions to be sure the GPU was completely covered in a thin but even layer, etc.). Within a few minutes of turning the computer on or waking it from sleep, the GPU temperature spikes VERY rapidly(well over 100C). This is driving a single 23" display, and even doing nothing but idling. I can also hear the display card fan running full blast. Any thoughts on what could be going on here?