2 850:

this sound like catalytic problem. try changing gas stations for now. try brand names. also check if you have the check engine light on and if so, try to read the code and determine if it is related to the O2 sensors.

beyond that you may need tune-up/adjustment to make sure the engine is not running rich as this would damage the catalytic converter.

yt850
1994 v855, 87k, 5spd, non turbo.