The cause for that could be a low transmission fluid level, a faulty vacuum modulator or no vacuum to the vacuum modulator, A faulty front pump assembly or torque converter, a loose valve body or a blown out valve body gasket, the forward or direct clutch pack has lost line pressure to the clutch pack (could be a plugged up transmission filter) or the frictions are burned up in that clutch pack, or the governor has malfunctioned, usually due to missing or stuck governor weights, or a stuck valve inside of the governor, a small plugged up line filter inside the governor fluid channel under the valve body, and most commonly because of a plastic drive gear at the end of the governor that can crack apart or will have sheared off teeth on the drive gear.
Check the transmission fluid level and inspect the vacuum line to the vacuum modulator for any damage and there should be full intake manifold vacuum to the vacuum modulator. Remove the vacuum line at the vacuum modulator and if transmission fluid drips from out of the vacuum line then the vacuum modulator is faulty and should be replaced to eliminate it as the problem, however there might still be a problem with the governor, the valve body, the forward clutch pack, or the torque converter and front pump assembly.
The valve body can be tightened down and the transmission fluid filter can be replaced, and the valve body and the spacer plate gaskets can be carefully replaced, and also inspect for a small inline screen filter inside of the governor fluid feed chanel if the valve body is removed. (there will be check-balls only experienced mechanics should attempt it
The torque converter, the front pump assembly, and the forward clutch pack can only be repaired by removing the transmission and except for the torque converter dis-assembling the transmission.
The governor can be removed without too much difficulty and the plastic drive gear should be inspected for any damage to the teeth on the governor drive gear and also the worm gear on the output shaft inside of the governor housing that drives the governor gear. Also check the governor or for any missing or stuck weights on the governor or for a stuck valve inside of the governor.
To remove the governor from the transmission remove the clip over the governor cover and then pry the governor cover off, then the governor will pull straight out of the governor housing. (The transmission cross-member might have to be removed on some vehicles to lower the transmission enough to remove the governor
Here are some images to help assist you inspect the governor, and let me know if you require any further assistance.