2004 Chevy Cavalier Engine Revs Up

While driving at a constant rate of speed, whether in town (40 to 45 mph) or on interstate (65 to 70) the engine will rev up anywhere from 300 to 600 rpms then after a second or two drop back to normal. The car is not bogging down or anything, just kind of wants to take off with you. Feels a littly like car wants to go into overdrive like for passing. Problem is intermitten but getting worse.

Firestone checked it over, said it sounds like transmission. The car has a totally enclosed transmission, one of those with no dip stick to check fluid level.

The only thing they could see was there is a seal where the tranny fits with rear axle (gears? Etc) and that it has a slow leak there. They suggest pulling rear axle and repairing this seal then refilling tranny thru some small hole. They think tranny is low on fluid and that is why the car is reving up like that.

Does this sound right? Does anyone have any advice or other ideas?

No check engine light on. Everything else running smooth, car as a whole running smooth.