Corolla 1991 Transmission Problem

I am wriring for the first time.
My problem is as follows:
1991 Toyota Corolla
4 speed OD transmission.
While driving in Overdrive the transmission slips down to the next gear
and out of overdrive.
This normally happens when we are going down a long desending hill.
The only way to recover is to pull over --- stop---and then start again.
The problem is intermittent as it does not happen all the time
Any suggestions as the car is a 1991 and I would prefer to do the work myself and avoid a transmission shop.
Thanks for your help in adance