Your magneto is bad. When it gets hot it breaks down. A fairly simple fix. Get your Model, Type, and Code information and go to Amazon. The part should be under $40 even if you want the genuine factory article or about $20 for a Chinese knock-off that will probably do just as well. The replacement is an easy afternoon job for anyone with a basic tool set. I don't remember the exact specification, but you'll want to be pretty careful setting the air gap. It's been a couple of decades since I shut the shop down. Should be available with a Google search as long as you have the Model, Type, and Code number.