Newish oven powers itself off intermittently

The symptoms would seem to indicate a short caused by moisture on the circuit that supplies power to the fan.

Usually the top vent front panel is held in by two screws and or clips, Get those off and the bottom of this peace should un hook, unless there are screws there also. if you can get this far, the rest will be obvious.

Inside behind the display panel on a small circuit board.

The magnetron probably has a partial short pulling more current than normal as it runs, the other two items that give trouble is the high voltage diode, "sometimes cracks" to the high voltage capacitor, "usually shorts"

This sounds like a keypad problem, sometimes a short developes in the keypad controller causing all kinds of intermittent problems, on the back is the microprocessor circuit board which controls vital areas of the microwave oven. The cost to repair will be high and consideration should given to looking at a replacement.

Saw something like this happen when a magnetron gasket was bad and Rf energy was interfering with the control board. Could also be a connection problem in the power circuit, acting like it was unplugged momentarily. Heavy draw from the High Voltage circuit when in cook mde. Eric

Is it on a gfci plug? If it is reset that. If not I would recheck the circuit breaker again

Quite proberbly t/t motor.

I've had this problem twice with our microwave. The hint to where the problem lies is that the breaker blows when you open the door at times. Inside the microwave are three microswitches; primary and secondary interlock micro switches and an Interlock Monitor switch. The switches are supposed to shut down the microwave if it is running and the door is opened. But, I've found the upper primary switch can stick internal at times and when the door is opened the stuck switch will, because of the way it is wired as a safety switch, cause either the fuse or possibly the main circuit breaker for the oven to pop. The reason the stuck switch blows the breaker or fuse is when the door is opened the interlock switch will cause the neutral wire from the AC power to be applied to the Upper Primary Interlock switch which normally should be open when the door is opened, but if the switch is intermittent or the contacts weld themselves shut the neutral line is connected direct to the "hot" side of the power line through the fuse and thermostat. The first time my microwave failed the switch had melted the contacts together inside the switch and the second time the switch became intermittent and would blow the breaker to the over ever so often. I suspected the switch the second time this happened and utilizing an ohm meter I checked the switch several times by opening and closing the door and once in a while it would remain closed instead of opening when the door was opened. Of course while trouble shooting the power cord to the microwave must be disconnected and use all safety precautions when working around the high voltage areas inside the microwave. I wrote Jenn-Air about the bad switch and the way it is wired into the circuit, but never received any acknowledgement. I suspect these microswitches are under rated for the amount of current that passes through the switch, thus they overheat and eventually arc the contacts together. I hope this helps explain the intermittent problem and could explain many of the intermittent blown fuse problems I see in these internet help sessions

Have you ever replaced the fuse with Higher Amps fuse. There is a circuit diagram inside the cover please refer to that and ensure that the rating is same.