SOLVED: I had the same problem with a 3 year old GE Microwave. It comes down to a design flaw. I changed the capacitor and all three switches and still had the problem. The issue turned out to be the white plastic housing that holds the 3 switches. Over time the hooks from the door latches wear away where they meet the white housing. In my case the top hole wore down faster than the lower hole causing the switches to release at different times when the door is opened which will in turn send a power surge to blow the fuse. Attached a picture of the worn piece. Certainly check the continuity of the switches first. Second I would replace the plastic piece holding the switches. I say do those first because they are the most likely causes and you don’t have to take the microwave down to do it. Lastly, if you still have problems, I would change the capacitor (DANGER - capacitors hold a potentially deadly charge).