Kurt,
You have described perfectly a symptom of a sticking three way solenoid. Or more appropriately the linkage for the solenoid. Your transmission uses this solenoid to adjust the control pressure to suit conditions. Low for a smooth idle, Medium for normal driving and high for full throttle driving. If you flip up the carpet on the passenger side of the transmission tunnel you will see an access plate that will get you there. The solenoid is mounted on top of the transmission and the rod runs down the right side. a Phillips screwdriver, a little oil and some wiggling should be all you need.
Or you could just accelerate to full throttle 5 or 10 times to shake it loose until you can get to it..."Honest officer, this guy on the internet said..."