I service a lot of commercial units.
If it goes dead for a while during or after cooking then comes back on, the magnetron is probably overheating and causing the magnetron thermostat to open. Then when it cools, it closes the circuit and allows power through again.
If it's intermittent or random, it may be a bad connection, usually on the control board or a loose fuse holder, or even an intermittent fuse.
You should do a continuity test the fuse while it's in the holder (with the microwave unplugged, of course) then turn the fuse by hand or take it out and put it back in, then test it again.
If it went dead almost immediately after pressing the Start pad, that's usually a shorted high-voltage capacitor.
If it went dead a few seconds after pressing the Start pad, that's usually a failing high-voltage transformer.
If it goes dead or blows the breaker when you plug it in or open or close the door, then there's a problem with a door switch or door switch mount.
There should also be a "mini-manual" hidden inside the unit behind the control panel, which is very helpful when troubleshooting.
Please feel free to contact me directly if you need more help. If you can scan and e-mail the mini-manual, that would help me help you, too.
We're happy to help and we appreciate your thoughtful rating of our answer.William E. Miller, AS-EET
[email protected]http://www.microwavedisplay.com