Question about a heater on a '91 Prelude

The heater on my Prelude went out a little while ago but has been going in and out lately and I'm not sure what the problem is. The rear defrost works all of the time but when the heater is not working none of the front heat works. This may sound weird but sometimes when I go over speedbumps the heater will start to work but won't work once I shut the car off then start it up again. Does anyone have any suggestions on what it might be? Thanks!

you might have a bad connection which would be why it works but goes in and out if u know anything about your heater take it apart and reconnect the electrical wires....its worth a shot if not 45 bucks will get it checked out the mechanic will tell u whats wrong with it