hard shift

my 95 sts sometimes shifts hard and the other day it would not shift at all and the service engine light came on. after pulling over and shutting it off for a min. the light was out and it shifted fine. any ideas what could cause this.:shakehead

That sounds like one of the shift solenoids is going bad. I'd have it checked out. AutoZone can check the engine codes for free, which is why the Service Engine light came on.