I am having exactly the same problem with my 1991 Corsica this weekend. I have read elsewhere that the crank position sensor can do this so my mechanic has started to take it out when it broke in two. He needs to remove the oil pan to push it out from inside now. The other possibility is the EGR valve however I wont know that until after the crank sensor replacement. PS not I'm not a pro but I will let you know in a few days. Answer the crank angle sensor on 3.1 and 2.8Liter V6's is prone to damage from stones and mud flinging up from tires in 4x4's and wears out and fails quite easily apparently. My COrsica had the very same problem last week and this repair did the trick and the car runs like new. When replacing crank sensor, it helps to apply heat to the sensor, not a little heat but do not melt it. The "O" ring sets up and makes it nearly impossible to pull out without breaking......leading to more problems than you bargain for. Once loose gently twist back and forth and pull at the same time gently. This helps alot. Ours had the same problem, turned out to be the ignition coil pack, located under the exhaust manifold, gets hot, opens up, cools down runs fine!
EDIT: Catalytic Converter as most of the problems listed here are.
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