I have a 1999 Volvo S80, 2.9L engine and it recently started giving me this problem. I was driving along everything good and then suddenly the engine started to run very rough and rich with reduced power. When this happened the message appeared in the message center 'Engine System maintenance Required'. If the car is driven long enough in this condition, the vacuum assist for the brakes is lost. In order to get the car to run normally, I had to take the cover off the throttle body and push the butterfly back to the closed position. The butterfly was stuck open when I removed the cover. This problem has occurred 3 times and each time the cruise control was being used. I have driven the car since without the cruise without any problem.

I have done a little research on this problem and it seems it could be problem with either the throttle position sensor or the throttle module. Any help with this will be appreciated.