re: #11 Charles Johnson

Charles:

If you look inside the microwave, there will be a mica panel which is where the microwaves from the magnetron enter the oven.

It will be on the top or side with the controls. Probably about 3 inches square.

If it gets dirty, the microwave will behave like you describe. Clean it carefully. It should also be removable to clean it if needed.

Should fix your problem.