I hate to resort to the old "I had this issue, try this" way of troubleshooting, but I can't help myself to relate to a similar issue I had on a 94 Olds 88 Royale.
The thread is long, but you may find some value in it:
After the car was given to a friend, he was relentless and determining the issue. Of all things, it ended up being the harmonic balancer was faulty. He had it replaced and the car runs like a champ now. I last saw him in November and the car had 205,000 miles and ran great.
I'm not immediatly jumping to the conclusion of a faulty HB. However, I would closely examine the rubber boot around the perimeter for cracks.
That PCM from Don (jr's3800) has made it'* way around! It was a troubleshooting step for me, too!