Transmission slips- is my control module bad?

I have a 1992 Mitsubishi Galant, and the transmission slips when it wants to go into overdrive (4th). I've been driving it with the overdrive on off, and it's been fine. If I'm above 40mph, I can press the on and it will go into 4th and drive perfectly. Some websites have mentioned that some Mitsubishi transmission control units are prone to capacitor leaking- could that be my problem, or is it more likely a bad solenoid, or another internal problem with the transmission? I've been trying to look inside the transmission control unit, but it's not easy to get it from under the dash. Thanks!