I had a similar problem with a right slick mag that turned out to be the coil. It worked fine when cool but once shut down for a short while it would heat soak and short out and then be fine once again when cooled. It was a challenge since it would cool down by the time the tools were out and the cowl was off. With high coil and rebuilt cost- a call to Aircraft Spruce and then back flying in a day with a new mag.
Scott- 2017 dues paid
RV-12 #120515, 319RV 175 hours.
1st flight 12/08/15, flew SNF, Oshkosh, Triple Tree & SERFI in year 1!
RV-9A, 536 hours, sold