Our GE Microwave and oven combo has developed a problem. Over the last month or so, the microwave is taking longer to defrost or cook items. Say a bagle that would defrost in 15 seconds is not taking 30 seconds. It is slowly getting worse. I suspect it is the magnetron and a new one is available for $120, but I don't want to replace it without knowing for certain this is the problem. I can only think it is the magnetron or power supply. I would think the power supply would simply fail, not fade away.

Any advice on how to diaganose or can someone confirm my suspicions?

Thanks