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.
2014 dues paid
RV-12 N95HW Purchased flying, RV-12 building
RV-9A, 536 hours sold, RV-7 project sold