Why does my LG Microwave heat only intermittently?

Over the range model. The microwave seems to turn on only once every 3 or so tries. When you enter a time and hit ‘start’ or hit the ‘30 Sec’ button, the timer counts down as usual, but the carousel doesn’t turn and the microwave doesn’t heat. Hit ‘cancel’ and try again once or twice, and the thing suddenly works. Maybe a faulty relay or contactor?

This is the second LG that has gone south on us, we’d like to fix it, but if not, we’re replacing with a different brand.