Ok I will have the codes read and get right back with you.
Are you positive you have an overdrive trans? If you do have the 41TE overdrive trans there would have been codes stored for sure. The picture I posted a few replies ago shows what the 41TE looks like. If you have a three speed trans then that would make sense why there were no codes.
With either trans the recommended fluid is ATF+4, you can use ATF+3 if +4 isn't available.
Ok. You don't have a 41TE overdrive trans, you have the 31TH three speed. It's not electronically controlled, it's all self contained and hydraulically controlled. You either have a line pressure problem, the front clutch is slipping, or the governor is sticking. These have a history of governor problems. It's going to need some hydraulic tests done with a pressure gauge while it's acting up.
An auxiliary cooler may help the problem if the cooler in the radiator is plugged and causing the trans to overheat. This does happen sometimes and will definitely cause your problem. The cooler in the radiator can be flushed and flow checked with a machine to see if it's blocked.
You're welcome, and good luck!