If the RPM is fluctuating without the vehicle speed actually changing, then it sounds like either it's shifting in and out of OD, or the TC is locking/unlocking.

the PCM is programmed to lock/unlock the TC whenever you lift on the throttle, so if the TPS is bad like the other suggested, then the computer is struggling to see what the throttle is doing, so it's locking/unlocking the torque converter, which is what's giving you the RPM fluctuations.