The microwave shuts down (looses all power) if it is used for more than about 4 minutes at a time. Could it be overheating? There is not a strange odor. But it does make a noise which can best be described as straining to work or rubbing, and does so even if the turntable is off. We have tried cleaning the area accessible when the top vent is open; there was a lot of dust, but it did not make a difference. The power will eventually be restored after 30 - 60 minutes. It does not seem to have any trouble with smaller time increments, unless the accumulated time adds up to several minutes. The automatic sensor feature also seems to not be working; initially we thought it was because the items were requiring longer cooking times, but then we remembered that this really was the first indication that things were not working correctly. Do you have any ideas about what could be happening?
Thanks for your assistance

Thank you Gene; a bad cooling fan does seem like it might just be the problem. Is this something you suggest is manageable for a DIY homeowner or time to call in a professional?
We have looked at our unit (which is mounted under a cabinet, over the stove and not on a countertop) and are having difficulty imagining how to open the control panel. Can you provide a few pointers for opening it? Thanks again for your insightful reply.