I bought a used 86 V8 Mustang T5 for my 65 fastback. I removed cover to inspect gears visually. Everything looked exceptionally well for a 20 year old tranny. Seems to shift OK on the bench but when installed for a road test it doesn't seem to want to downshift from 5th to 4th without first going to 3rd. This happens only when the car is in motion. Once stopped it will go from 5th to 4th. . Anything I can do?