It may be a bad connection on the fuse holder, an intermittent fuse, a bad connection on one of the thermostats or elsewhere, or a bad connection on the control circuit board.
If you want to contact me privately with the model number, I can give you or a friend free DIY troubleshooting help and I also do microwave control circuit board repairs.
For more free DIY and other help and advice, feel free to contact me via my site.
Always include your FULL model number*, FCC ID number*, and detailed symptom information.
* from the tag in the cooking cavity or elsewhere on the oven (or send a clear photo of tag)
William E. Miller, AS-EET
http://www.microwavecontrol.com
Microwave Oven Control Panel Repair Nationwide
"Recycling by repairing since 1982"