Im not sure the exact model oof tranny you have but id assume it is the AW70L which it should for that year and make, the problem you are having is cause by a woren down nylon/rubber ball in the valve body, this is and easy fix if you know how to pull the valve body and tear it apart and reassemble it, otherwise you will most likley have to have someone do it for you, i did it on my 89 fixed all problems including the one your decribing, the only pain i had was getting the balls to stay in place while i put the body back together, but with some deep breaths i was able to get it done, you dont even have to drop the tranny to get the valve body out, just remove the tranny pan and unbolt the body, remember to undo the kickdown cable,