Diagnosis
The waveguide cover is damaged or has accumulated conductive carbonized food deposits. The magnetron's energy is arcing to these points.
How to Fix It
Unplug the microwave. Remove the waveguide cover (the rectangular metal or mica plate on the interior wall). Inspect for burns, holes, or a thick layer of grease. Clean it thoroughly with a soft cloth and mild detergent. If damaged, it must be replaced.
If That Didn't Work
A damaged stirrer fan or its housing can cause arcing. Inspect the plastic stirrer assembly in the top of the cavity for melted spots or debris.
Heads Up
Never operate the microwave if you see arcing. It can damage the magnetron. The waveguide cover is a critical safety component; never run the unit without it properly installed.