'92 Taurus transmission trouble

I have a '92 Taurus (3.8 L with axod tranny) that has occasionally been downshifting (probably to 2nd gear) at speeds from 25 to 55 mph. Recently it would not upshift after one of these fits until I turned off the ignition and restarted. Everything went back to normal. Does anyone know what problem might be causing this?

I've already checked the transmission fluid & all is ok.

I had similar problems with my 91 3.0L...so i hooked it up to a diognostic machine, and it said my (TPS) throttle position sensor was bad, I changed it and haven't had a problem since........you might want to try it out.