Dodge: 91 Spirit Trans Problem

I have a 1991 Dodge Spirit 114,000 mile, V6 with automatic transmission.
The other day the trans started to knock itself down into 1st gear while
driving after the engine warms up. When cold the trans seems to shift ok.

If I shut the car off and restart it while warmed up the trans will shift
normally until 35mph. It kicks in and out of gear at that point. If I'm
lucky enough to get it up to 45mph after a restart, the trans slams back
into 1st gear instead of overdrive.

The fluid level is full, clean and not burned. The filter was changed about
4,500 miles ago.

I'm tired of peeling my face off the windshield, any thoughts?