Volvo 240 GL Transmission

You have an overdrive relay issue. The tansmission is really a four-speed box, but the OD selector either locks our fourth gear or allows the car to shift normally into fourth. I use the OD on my volvo all the time, because the revs drop in half when the OD is activated, The relay is located behind the glove box, toward the center console. $10 buck on Ebay and your transmission should shift correctly again. I would try that BEFORE more expensive fixes, like OD solenoid $200 or transmission control module. Just for kicks a picked up a spare ECU on Ebay for $10 because these get costly if and when they go. Easy fix, though, a couple of plugs.