Just out of curiosity, did you encounter the three minutes and you're out problem only at one location,
No it has happened in 10-15 different states. In fact, Indiana is the only state we've ever been in where it doesn't
Also, are you always running your tests with a cup of water or something else in the oven, as they do not appreciate being run empty?
We don't run tests per se, we cook food. (Even the PC folks had a "test burrito" or something that they were incinerating.) So there is always something in there.
There's no evidence of low voltage conditions elsewhere in the coach. No lights dim, when the problem happens or at other times. The oven just beeps and resets. You can immediately enter the cook time minus three minutes and it will pick up where it left off, for another three minutes.
Also, the problem appears NOT to happen in convection mode, which uses vastly more power (and generates vastly more heat) than microwave mode. So, in my opinion, the problem is unlikely to be power or heat related.