MAX89'

my 89 gxe has the digital heater control unit. here's the problem. everything can be working fine and then the thing just turns off, all the lights go out and fans stop until i hit the thing. most of the time after beating it for a sec the unit will light up and hot or cold air starts blowing again. the funny thing is that it works perfectly when on and it just completely dies sporadically. i have no idea how to fix it, any suggestions would be appreciated.