89 lxi coupe 4th & rev. intermit. same time

[shotgun]

89 lxi coupe 4th & rev. intermit. at same time.
When 4th @R are working they engage strong and don't slip they also seem to stay untill car is turned off.other than that there is no pattern to the failure. 4th is irritating but reverse is damned annoying! if any body has sssome good clues I would really appreciate it !
(fuel injected automatic)