What went wrong?

[BGlodde]

Here's my series of events (I searched):
1) Trans Fail Message a few days back, but nothing unusual happening. It threw this when I turned the X off after a brief drive. I got nervous about it, restarted it and drove around the block to see if the code came up again. Nothing displayed this time.
2) Yesterday, I cautiously drove it to a destination 25 miles away, nothing unusual. No codes. Completely uneventful.
3) On the way home (after approximately 15 minutes of driving), the CEL illuminates and, when accelerating or keeping the speed up on the highway, the X vibrates and the accelerator feels like someone shoved a sock in the exhaust (only way I can describe it). Now, when it is idling, it's very choppy, even from a cold start.

I am going to take it to the dealer for diagnosis, but would like to gather some informed opinions in case it's something minor I can do myself.

It's a 2003 3.0i with 68k and is not driven daily.

Any clues?