Erratic behavior may sometimes be traced to intermittent
grounds or power supplies going bad.

I've seen this in other electronics (not microwave ovens).

You said the problems were intermittent. Is the oven more likely to work
correctly when it's cold (that is, it hasn't been used for several hours),
or do the problems occur at any time, whether or not the oven has been used?

If the oven usually works correctly when "cold", I would start looking at
the power supply, especially the capacitors.