[cheerios1]

Before narrowing it down to the tranny...check the idler, reason I say that I had combustion issues. EGR valve giving me the code, cleaned intake areas & everything I could think of and finally when i was done, at least thinking i was, my 98 malibu started idling funny. (this was all happening to me 2 weeks ago)
Revs at a stop light would go up and down and like your situation, I would put it to park then all would be okay. When it was time to go, it would jerk a little at take off, (never paid attn. to the speed) and as i picked up speed i was fine.
I stopped at an Advanced Auto they recognized the issue by my description. They showed me where the idler was located, cost me $9 bucks. I believe it's also reffered to as an IAT temparature sensor in the intake area after the filter (u have a 99, i don't think it's any different). When I replaced that sensor, my jerking stopped asap...
It's worth a short....first things first though, stop by an Advanced or Auto Zone and see if they can recognize the issue and ask them if the idler could be what's causing the jerking.
Never asked you but do you have an engine light on???