I had the exact problem on my 1992 olds 88 a month ago and the GM dealer could not find any problems either. I posted my problem on another car site and two people (only two people) both said the problem was my crank sensor. I took the car back into the dealer and said to replace the crank sensor and he said that he seriously doubt that could be it but he would replace it.
Sure enough, that was the problem and it has been running fine ever since! Total charge was like $170!
Let me know if that is the problem - I am curious?
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