Try to find where the arcing is coming from by viewing th e inside of the oven "cavity" while the problem is happening. You can safely run a microwave oven with a container of water inside (something like a margerine container for example). The arcing usually indicates that a component is breaking down and carbonizing, therefore becoming conductive and sparking while the oven is on.
It is common for the "waveguide cover" to do this. The problem is caused initially by food debris, etc. becoming stuck in the crevices of the cavity and then becoming conductive. When you locate what is arcing, you will know what component needs to be replaced. Jim