Had the same symptoms. Difficulty in starting became worse. Mag check was ok, when I could get it started. Checked fuel flow, spark plugs, mag timing, ignition switch, and mag grounding. Turned out to be a bad left mag. Overhaul shop tested it both hot and cold. Coil was bad and points were not set properly. I had the mag rebuilt and solved the starting problem. Mag only had about 310 hours.
Check all the easy stuff first before pulling the mag. Mag overhaul will cost $$$.