I had a simular problem. The coils in my 4hp had small cracks in them. When the engine warmed up to operating temp, the spark voltage took the path of least resistance to ground - threw the cracks in the coil, not to the spark plug. This was an intermittant problem. Drove me crazy. These cracks are so small you can hardly see them with your eye. I replaced both coils, both points and both spark plug wires. Problem solved!