Craftsman 20 hp B&S 42 cut 5 yr old Auto trans

craftsman 20 hp B&S 42 cut 5 yr old Auto trans Last yr started to backfire and lost power for a few min after 2 or 3 hrs of running I plug wet 1 plug dry installed new magneto runs ok but sometimes wont start at all Then out of no where starts right up Ant ideas It seems like an electrical prob.When it wont start it just floods seems like no spark had this prob now 1 year.I thuoght maybe get second magneto and new wiring harness