won't change to 4th just revs up

Gears change viciously from 1st to 2nd and to 3rd. Then won't change to 4th just revs up to 4.5 then changes some times to 4th. Few years ago the alternator blew up when in for a service. We replaced it with a generic alternator. Could the computer be playing with the gears because of the generic alternator.