it's been many years, but i'm another victim of having this same problem being caused by a bad pickup coil. mine would only act up when hot. you can check the resistance of the pickup with a digital multimeter(i don't remember offhand what the range/specs. are, but the info is out there) & mine would be in spec. when cold, but out of spec. when hot. a couple of times it started with a huge backfire from all the fuel in the exhaust from cranking for so long!