1994 Trooper 3.2 / 4 door / Transmission problem

I have a "94" 3.2 Trooper with 185,000 miles. Just this week my mother-in-law borrowed it and was on her way home and called me about it not shifting correctly.

I test drove it and it did act up in overdrive. The next day I changed the transmission filter and fluid. Drove it 80 miles to a friends house. No problem.
Hooked up to his boat and about 20 miles down the road it started the same thing again.

The owners manual says that the computer goes into a "backup" mode and the transmission has to be shifted manually. It worked. "Check Trans" light came on and then reset after I turned the vehicle off. Doesn't do it all the time. Doent' have any problems shifting when you first start out driving for the day. Only after it's got warmed up.

When it happens. There is no drive to the transmission. You can give it the gas and the RPM's go up but no drive. Acts like you pulled it out of gear and shifted into neutral. Manually downshift and it starts to work again.

Need some help on this one please.