4T60-E transmission kicks out of overdrive after warmed up?

I have a 1994 olds cutl supreme with the 4T60-E trans 3.1L engine,when i 1st bought it was ok until about a month after that it would shift ok but after it warmed up it kicked out of overdrive into drive,i drove it like that for about 2 years finally the trans went out so i got another one and replaced recently thinking that would cure it,but it still does it, guess its possible that 2 transmissions could have the same problem but it leads me to believe that its could possibly be a bad cell in the computer, the service engine light is not on either by the way