Find one of the better scanners that measures things like TCC slip, transmission duty cycle, and the like. Take a drive and post your results here during normal load, and during one of these episodes. It'll help us out a lot, and make you feel more like a gearhead.
Some more things to ponder:
- What color is your transmission fluid? How does it smell? Ideally, it should be close to bubblegum pink with no odor. Red (or worse, black) and/or a burnt smell is bad news.
- Speaking of which, when was the last time it was changed?
- Does this happen when the transmission is cold? Hot? Always?
- Have you ever had any transmission-related CE lights? (Mostly P17xx and P18xx)