4850 overheat with Accelero S1?

I'm having some problems with my 4850 since I put the Accelero S1 rev2 on it.

I have heatsinks on all the ram chips and the VRMs, and I have left it passively cooled for now. The problem I'm having is that I'm getting a black screen after a short period (around 20mins) or graphics intensive games.

I can reproduce the problem easily using the crysis benchmarks. I did this while running GPU-Z to record the temperatures. The last readings before the crash were:

GPU: 75.0
DISPIO: 74.5
MEMIO: 84.5
SHADERCORE: 78.0

These don't seem to be particularly high to me, so my question is, is it still possible that my card is overheating? If not, what else could it be? How do I troubleshoot this further?

Any help much appreciated.