1990 Cadillac Fleetwood Brougham transmission problem ?

Hello friends, question,  i took my 90 Fleetwood on a cruise, it wouldnt go past
35, suddenly felt like its in neutral, i stepped on pedal but nothing, while rolling
i switched to neutral and back to Drive, it engaged for few minutes and
suddenly same again like i am in  neutral, ,i pulled over, put in D, drove off ,
went up to 45 mph but slowed down again on its own, then it still wouldnt go
past 20 mph this time, ,Trans oil is full, had to drive home 20 mph,  any ideas ?  
or is the whole transmission gone ?    thanks for any help.