Welcome to the Club!!!
I assume the trans works good, as in no slippage. Depending upon mileage, oil pump could be worn out. You can have the trans shop do a pressure check to verify.
Lift hood, at the rear of the engine on the intake, you will find a vacuum hose, approx 1/8 inch inside diameter, going to the rear of the car disappearing below. This hose connects to the vacuum modulator on the trans.
Pull the hose off from the intake, and either manually suck on it or apply a vacuum hand pump to see if it holds a vacuum. If it does not, then either the hose/tubing is faulty or the vacuum modulator needs replacing.
If the above doesn't fix the problem, then I would suggest a removal of the pan and valve body for a cleaning. Maybe even remove the valves from the valve body, one by one, and lightly sand them with emery paper. BrakeKleen works great for cleaning.
Upon installation, be sure manual valve is hooked into the shift lever. Clean the pan, install a clean filter and gasket, and fill with fresh fluid, DEXRON III.
Checking & repairing the governor requires removing the rear extension housing, after unbolting and removing the trans cross member. BE SURE TO SUPPORT THE ENGINE/TRANS FROM FALLING. Could really make for a bad day. Then, remove the speedometer driving gear with a long jaw (15 inches) gear puller. Then, remove a lock bolt and pull off the governor assembly and clean it or replace with a known good unit if worn parts are found. Replace in reverse order.
If you would like, remove the bottom cooling oil line from the radiator, attach a length of hose to the cooling line, direct this hose into a drain pan, and have an assistant start the engine. The pump will pump clean fluid from the pan thru the torque convertor out the cooling tube. When bubbles appear, turn off engine, add a few more quarts of ATF, repeat again. When fresh fluid comes out, stop engine, re-attach cooling tube to radiator, top off the trans fluid. And keep fingers crossed....
Others here may have some suggestions.
Everett "OBJECTS IN THE MIRROR DISAPPEAR UPON RAPID ACCELERATION"