My daughter's 99 Mazda Protege with auto trans has started acting up when in overdrive. Between about 40 and 50 mph it will suddenly either downshift or shift into neutral for about a half second and then go back into OD. It's hard to tell exactly what's happening but it suddenly revs up and feels like a downshift but it could be going into neutral or just dropping down a gear. It only happens when overdrive is on. Sometimes it will happen several times within a short time and sometimes it will go for days without happening. I can't see any pattern to it. Above 50 mph it's fine. The OD indicator in the dash does not come on when this happens like it does when you turn off OD. Any ideas what this could be? Vacuum line? Electrical problem? Solenoid? The fluid has been changed on schedule (car has about 80k miles on it) and looks normal. Any info will be much appreciated...