I agree and disagree with you.
First, I agree that GM makes good cars. But I have had best luck with Pontiacs, they just keep going. My next car is either going to be a 2001 Bonneville or a 1999 BMW 740. I have driven both and would compare them fairly equal.
The problem with technology is that there are lots of things to go wrong. I do not know many other cars that get 31MPG on the highway. But the Bonne does due to its lock up torque converter and TCC solenoid. Some Japanese 4 bangers would love to get 31Mpg, and American 4Cyl cars like a Cavalier only get 25Mpg. That sux.
So yes, reliablity can be an issue when they keep designing new technology. The 81 Buick Hydromatic transmission is not real high tech but it works well.
I have a friend that was looking at buying a car and has been buying Japanese cars. I said that Pontiacs are great, as long as you dont mind little things going wrong. Its always something small thats a problem, but not usually anything that stops the car. They just keep on going forever.
Second, I am investigating the fact that it is a trans issue here. I dont want to admit it though, coz that means real money. I am trying to get my hands on 3 coil packs to replace those and see if it helps. But as you suggest, its probably the Torque Converter.
To answer your questions, I found that locking into D instead of OD didnt change things at all. It still does it in both positions.
I dont know about any additives. Some people swear by them, but I have never had many results. My mechanic put a "great" additive in my trans oil last change but it didnt do a thing to help this issue.