Ken,
I know the feeling/issue you are having from experience. It's common with higher mileage etc. It's either clutch slippage, FWD band slippage, or FWD band beginning to break. The reason it doesn't happen when you select manual 1 or manual 2 is because those engage a secondary band in addition to the FWD band, therefore adding more hold power.
I would suggest not full throttling from a stop and you'll find the trans will likely last quite some time. In my Regal the trans had this issue when I got it at 160K, I drove it to 186K before I pulled it out for a different issue.
