Sometimes the microwave works as normal.

However, much more frequently, there are times when the microwave will be powered on but not running and then it will spontaneously start running as if it's heating something (this seems to occur after recent use or most often after the door is closed after a legitimate use of the microwave). I don't believe during this time that it's actually heating anything, it just appears that it does, i.e. light inside microwave turns on, fan turns on, turntable turns. While this is going on the clock just shows the time, there is no countdown of time (I guess b/c it was never told to start). Pushing the Stop button on the control panel does not shut the running microwave off. The only easy way to do so is to push the door eject lever (in the lower right) and the opening of the door stops the microwave from running. I also noticed that the door doesn't have to literally open to stop the microwave from running, just a little push of the door eject lever stops the microwave from running and the door stays closed.

Obviously this is unsettling b/c it's a possible fire hazard. Any advice as to why the microwave would function like this? Is this a switch problem?

If you recommend a part, if possible, please include the part number so it's easy for me to order.

I think the microwave is repairable and I don't want to throw it away if it can still be put to good use (or maybe 13 years of service is enough; it did get used daily). I greatly appreciate any advice and help you can offer.

@Denman:

The full model number is Kenmore 565.68490790. I forgot the 790 on the end. Didn't see it on the doorjam; was going off the one in the manual.

Does the full model number change anything in your diagnosis?

Thanks.