I have a manual '09 GTS 957 that has been a great vehicle for years now EXCEPT for one very annoying reoccurring problem that I cannot seem to resolve.

At extreme heat and it appears at high altitude the vehicle experiences intermittent RPM loss. The higher the altitude the lower temperature the event appears to occur. For example at about 7000' the event will occur at about 92 degrees. At 2000' the event will occur at about 105 degrees. Allowing the car to cool as well as the outside temp returns the car to normal.

I used to think this was due to a faulty CRANK SHAFT POSITION SENSOR. I assumed under heat the part was failing and replaced it more times than I'd like to admit. My wife questioned the logic recently that the event was caused by this part as even a newly installed part would lead to the same event relatively soon.

The car runs perfectly in cooler temperatures at any elevation and great at high temps at low elevation.

I tried running lower octane fuel as I've experienced some finicky motoring in my Volvo 850 that an octane lowering fixed while at elevation. No such luck in the 957.

At the moment I'm at a loss. I did come across an old thread that touched upon this but no answers came out of it. The only thing I can think of now is the event is caused by incomplete combustion due to lower levels of oxygen as (as far as I know) hot air has a smaller amount of oxygen and as we climb in elevation we also experience less oxygen. Could it be that there's a failed sensor somewhere that would normally compensate for this?

Thanks in advance,

Justin