Excessive GPU heat

When I bought my VGA card back in April I decided to go with a specially cooled model of the evga ACS³ series. Because this series contains a special heatsink design, it can cope with higher GPU and VRAM clockings. The 8800GTX KO I bought contains a GPU clocked at 625MHz instead of 575MHz while the DDR3 memory is clocked at a juicy 2GHz instead of 1.35Ghz.

Now that it has been loyally pleasuring my visual needs up until now I started experiencing unexplainable crashes, especially while playing S.T.A.L.K.E.R. which is even more weird since Crysis was running smoothly. I basically tried everything: I installed XP, lowered the resolution and texture size, removed the game mods, installed all kinds of stable and beta drivers - all caused the game to freeze after 5 to 10 minutes followed by a BSOD after another 2 minutes which just told me that the graphics card was the root of it. I installed the same game on the PC of my girlfriend which happens to have almost the same VGA card (8800 GTS) and the same mainboard. Of course there it worked like a charm.
I finally found out that the card was massively overheating both while idling and under load (85C and 98C respectively) which is way too high, considering that I cleaned the dust off the fan regularly. Setting the fan manually to 100% didn't help but at least using an 18W/220V industrial fan remedied the situation but was unbearable noise-wise.

Frustrated and already sad about having to send it to RMA I carefully unscrewed the covering metal case of the board in a desperate move. What joyful sight that resulted in: The heatspreader was basically blocked by a wall of dust, making it impossible for the fan to send any air through that part:

Now everything's back to normal - except that my newly ordered mainboard turned out to be defective and waits to get replaced. No meddling with hardware until Christmas is over, sniff.