1994 Toyota Corolla Overdrive light flashing

My son has a 1994 Toyota Corolla and the overdrive light sometimes starts to flash and it feels like it is running in 1st gear. We left the car with our local Toyota dealer for a couple weeks because they were unable to pull up any codes when it wasn't flashing. They drove it off an on as time permited and couldn't get it to flash, although it seems to do it more after the car is warmed up so I don't think they were driving it long enough. They explained that if it is a computer problem, most of the computer stuff is located within the transmission itself and they just wanted to replace the transmission. Before my son commits to spending this kind of money, can you tell me if it is possibly something else? My husband did notice today that there is a cable under the hood, possibly a speed control cable, that seems to be very loose.