Problems with Overdrive 780

Hi, I have a 91' 780 with 172k. Auto transmission. Anyway, a couple
months back my car would drop out of overdrive while doing freeway
speeds. It was just temporary and only lasted a few seconds. Then a
few weeks ago the overdrive would go out completely, but was
intermittent with no real pattern of occurence. The "yellow arrow" on
the dash to shift up would stay off when this happened, though it turn
on/off if I pushed the button on the gear shifter.

I took it to my mechanic and we took it for a test drive. He also
diagnosed it as a overdrive problem. He assumed it was the solenoid,
but after leaving it at his shop he said that the solenoid was good.
Also, the trans fluid is full and still clean. He thinks the problem
is the transmission itself and suggested I take it to a trans shop.

My question is could it possibly be a bad OD relay? He said no and it
would appear to still work due to the fact that when the OD is out the
"up shift" arrow on the dash can still be turned on/off with the
button on the shifter. I am going to be taking a road trip soon and
would like to fix it soon beforehand. Any suggestions or tips?


thanks,
CTV