Reference the service bulletin DrJay posted in that 6-page thread... GM recognizes there have been cases of a "chuggle" that results when the TC locks and unlocks under load. I realize yours is completely unlocking and staying that way, but it may still be related to that same issue. The service bulletin indicates that the problem can be fixed by recalibrating the TC lockup parameters in the PCM. What do you think? I'd say its worth a try, unless you're fairly confident the problem lies elsewhere. It sure is cheaper than a new torque converter.

I was intending to have this issue addressed on my own car, but was plagued with other technical issues. Now I have an Intense PCM waiting to go into the car, and I'm betting that it will cure my surging/chuggle. Based on some of the computer problems we've had with our Olds 88 I have a "feeling" that I'm barking up the right tree. We'll find out how good my instincts are in a week or so!