IOH Heatsink error causing fan spin up to 4000RPM

Hi,

I am referring to this thread http://forums.macrumors.com/showthread.php?t=1457743

I upgraded my GPU to the ATI 5870 and observed intermittent Booster 1 fan spin ups to 4.000 RPM. I then thought it is an error due to the ATI, but using iStat I observed, that this fan speed up only happens, when the IOH Heatsink temperature suddenly exceeds 120°C for some seconds - running programs or in idle, independent of the GPU.

SMC reset didn't work.

So, I think the IOH Heatsink Temperature Sensor is broken?

Thanks for help!

Leif