This is usually a sign of a failing magnetron on these units OR a bad spot in the cavity.
Is the bright light in the middle or along the edge of the cover?
Middle and almost filling the cover
That is definitely the magnetron on this unit. Let me look that up and see what kind of cost you are looking at. How old is the unit?
It's more than 10 years old. I can get the Magnetron from Sears for $79