Automatic transmission shifting sporatically
 

My daughter has a 2006 MCS with an automatic transmission. The car has 75k miles and has never given her issues since she bought it with 50k on it. The transmission is NOT the CVT type and she only drives it in the standard drive mode, not manual shifting. There is no sport mode on this car. There have been no service lights at all. On Tuesday after getting gas (always 92 oct good gas stations) she got on the freeway and at cruising speed the transmission just started shifting up and down all on it's own. You can watch the tach go from 3k to 4k to 5k and then jump back down and sometimes the shifting going down in gears is very abrupt. It stops after a period of time but comes back again. Also if you pull over, turn off the car and restart it it seems to stop for a while. I drove it myself last night and it took about 5 miles on city streets before it started doing it. and then I got on the freeway and got the same result she has been getting since Tuesday. Anyone have any ideas what this could be? Again, no codes have been thrown as there has been no service lights on at all. She did have her boyfriend in town for the last 6 weeks and he had been driving the car while she was at work but this issue did not crop up until the day he was leaving to go to the airport. I have been told on my own MCS that the transmission can get "confused" at times when different people drive the car because of different driving styles??? Is this a possibility? Any input is appreciated.