How old is your microwave? Perhaps a better question is, does it still (heck, did it ever) heat evenly? Here is a quick way to check:
Put some marshmallows on a microwave safe plate that fills the microwave as much as possible. Full size marshmallows works better because we want to cover the most surface area, but regardless, just make sure you use the same size.
Put the plate in the microwave and let them cook.
When the marshmallows are done look at them. Hot areas burn first and cold areas melt last.
If you can stop your microwave from automatically rotating your food than you may want to try this rotating and not rotating. The higher quality the microwave the less rotating will have an impact.