Transmission problem, but not really?

I have a 99 Taurus 3.0 Vulcan. The problem is the transmission act like it goes into neutral or slips between the 1-2 shift. The problem only occurs when the engine is warmed up, never when cold. The original transmission was rebuilt, then another installed and then yet another installed and yet the same problem. Other items replaced include: plugs, wires, pcv, air filter, fuel filter, pcm, tps. No codes. The problem remains. Any ideas? :banghead:

Possibly a speed sensor (input or output shaft) or fluid condition/level.

-Rod

The trans was rebuilt professionally. All sensors, solenoids, servos, valve body have been changed. The trans currently in the car was a known good unit.

My Taurus does the same thing too. Been doing it since 75,000 miles and now has 161,000 miles so it cant be too serious. I am going to do a fluid transfer this spring and add some Lucas trans additive, worth a try.