This below cabinet mount Spacemaker microwave has all functions working fine, and there is no noise difference when it doesn't heat compared to when it did. Every now and then it would not heat, but trying different controls over again, would usually result in it heating. Then it heated fine for a couple of days. Now, although everything else works, I can no longer get it to heat. What is the most likely component(s) failure given the above symptoms? I suspect the magnetron, but can they intermittently fail? Thanks-