Long time ago, I had a similar problem. So I bought a new one and gave the old one to my neighbour's sone who could use the parts. This clever boy checked the big condensators and found a bad one. He bought a new condensator in an electronics shop for 50 euros and as a result had a almost new microwave. These electrolite-condensators have a much shorter life than most electronic parts.Remove the power from the apparate and leave it half an hour to let the power drop from the
condensators. Then check them for burns or bubbels.
Good luck !
P.Klaver, in Holland., March 2008