Chuggles up minor hills when in 4th gear.
Overdrive kicks in and out.
Harsh.
Kick the throttle, get up the hill, then everything returns to normal.

What'* causing this?

You really need to get a scanner on the car to see if the computer is commanding the car to shift the way it is doing. If it is not, the you have an internal tranny problem. If it is, then you need to see which sensor is not giving proper readings.
Any check engine lights coming on?

I have a Scangauge II plugged in. No codes.

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)