I have a 1994 S320 with a transmission that has a mind of its own. I need help trouble shooting this issue and I feel the more people I ask someone will know just what is happening:

When I am driving between 80-85mph, on a decline, the transmission will just go into neutral. No acceleration or deceleration just a steady speed.

If I gradually push the on the gas the transmission will NOT go into overdrive, it will go into 4th or 3rd gear and the rpm's just go up.

I have to TAP the gas (as if I am trying to get the passing gear) then the transmission will go into overdrive however it will not stay there. It will go back and forward into neutral or 4th gear as long as the car is traveling about 80-85mph. faster or slower no problems shifting.

Once this happens I have to slow down, pull over to the side of the road and stop.

When I start driving again the transmission shifts perfectly!

I have no clue what is going on with this transmission because it does everything I ask of it, MEANING:

No clunks, pings, clanks, oil leaks or grinding from the transmission and it has the right amount of fluid.

unnoticeable, smooth shifts coming up to speed on surface streets.

smooth powerful shifts on hard acceleration ( will do a burnout if I hold the break)

holds reverse (went an entire city block up hill no problem)

came to a complete stop on a hill and started again transmission shifted no problems.

here is the caveat it will NOT do it when the mechanic test drives it! He will fix it he just keeps asking me "what do I want him to fix?"

DOES ANYONE HAVE ANY IDEA WHAT IS GOING ON WITH THIS TRANSMISSION?