Fan not kicking in most of the time for GE Microwave Model JKP78GOK?

Unit from 1988.

Fan is not working sometimes. (Food is getting warm) Replaced fuse. Checked both door switches, they are ok. Sent out smart card, they said it was ok. Tapping on front display, unit will work sometimes. Turning main breaker of helps some times, but unit works only once or twice. What can be wrong??