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Hi, that usually indicates a faulty mag tube. The top of the mag burns and causes the arch. Sometimes it will continue to arch and other it completely stops. You need a new magnetron. Check the use and care guide some of these units had factory extended warranties on the mag tubes. If you're out of that, the average cost to fix is roughly $300. You need high voltage experience to replace the mag. A microwave can hold lethal voltage even unplugged. I work on the honor system here @ Just Answers. Please remember to rate my answer positively before logging off. Thanks!