Microwave won't heat for first minute - starts working normally after that.

I have a 2 year old Kitchenaid Microwave/Convection oven model KHMC1857WSS that has developed a strange problem when using the microwave function. When set to 100% power the microwave does not start heating until after a full minute passes. At 61 seconds the fan slows very slightly and it starts heating normally. It will also heat OK at 90% power. I've checked the voltage at the wall plug and it's fine, even under load. I've done some research and found solutions posted saying that the magnetron is energized by the control board and that only the control board could cause a problem of this type. I ordered a new control board and after replacing it the microwave has exactly the same problem. What else can cause this problem?