I think this is an electricity question: When I start my microwave, it starts normally the motor revs down drastically, then back up to normal, then stops. ..a total of about 10-15 seconds. Thinking the microwave was shot, I bought a new one. Same results. What could be the cause?
What you are observing is the klystron (or other magnetron) tube (the microwave generator) being cycled on and off. Most home microwave ovens cycle the generator on and off to allow the heated regions of the food to disperse the heat more evenly throughout the entire mass. In fact, on those models where you select "%heating" the duty cycle changes, not the actual output power.
Next time, before tossing your microwave, observe the results: does it boil a cup of water as quickly as it did when new? Basically, unless it doesn't heat the water at all, it's still working.