2000 Grand Prix GTP transmission issues?

My dear Pontiac friends, the time has come for me to ask a question of you. The car in question is my 2000 GTP sedan. Recently, I noticed that my car was a bit jerky in reverse, but only for a few moments. I attributed this to the fact that my oil was due for a change (I decided to go by the DIC for a change) and since the problem seemed to stop after the change yesterday, I assumed that was it. Today, I felt as though I was revving a bit higher than normal at speed. After making a few complete stops at lights and getting back to full speed, I decided that I was definitely running at a higher RPM than normal. After going a half mile more, I suddenly realized that I was going 30, and that going at close to WOT resulted in very slow acceleration. Long story short, it got worse in the next mile and a half and ended with me stopping. All gears, including reverse, resulted in revving like I was in neutral. I shut the car off and looked under the hood. I restarted and the car drove, but after 15 seconds or so, it slipped back into neutral. I limped to a parking spot, and determined the following things:

1. After being off for roughly 25 seconds, the car can shift from neutral.

2. Once in gear, it takes about 15 seconds to slip back out.

3. Once slippage occurs, there is no way to get into gear without shutting the car off and repeating the process.

4. The shift lever moves freely, but there is no sound of the car changing gears.

The engine runs fine, and the fact that I can get into gear at all, albeit for a few seconds, is making me think that the problem is minor. I'm almost sure it's electrical, since shutting the car off seems to rezet it. Anybody have any thoughts? I know Tony has spoken of his GTP before, so mayne he has some thoughts? My GM tech buddy will be checking it out in a half hour, and I'll be getting it to a tranny shop soon, but I just want to have some idea what's going on here.