Title says it all. I drive about 44 miles a day to and from work and drive at 65 mph consistently for about 10-15 minutes and it will drop into overdrive without the ability to take it out. Sometimes it will do it and sometimes it won't. After going a mile or two, the transmission will go back to normal. Faulty solenoid?

Engine and transmission have 180K on them. I bought it locally a few months ago and have been slowly replacing the fluids on it since the guy didn't take care of it. Transmission fluid, filter and gasket have been replaced.

1990 Volvo Turbo Wagon

 

I have had the wires that run through the shifter to the o/d button fail,
due to fatigue from shifting,
which would be easy to check for continuity
with the help of an assistant.

 

I assume you have the AW71 automatic right? What you mean is it drops OUT of overdrive and revs go up, yes? As was stated above, those switch wires occasionally fray and lose connection. However, my experience is that what happens even more often is the single wire going from the overdrive solenoid on the side of the transmission has become oil soaked and frays and eventually breaks. Get your car on a lift or jack stands and look for the solenoid and follow the wire.

 

The Relay in my 940 blew out and I had the same problem (OD would randomly turn off mainly on highway), until it finally wouldn't even go into overdrive anymore.

Bought a replacement relay for like 20 bucks? and plugged it into place and problem was solved. I forget where the relay sits, somewhere in the dash i believe was the part I replaced

 

On the 740's the OD relay is in the relay panel behind the fuse board. Pull the ashtray, the piece behind it and all is revealed.