Obviously it could be host of things but I'm guessing you're just relatively confident of the fuel. That being said Heet is a good product and start. Review how long it's been since the distributor Cap has been changed. If the gasket has disintegrated moisture could be creeping in. After that I think you start becoming more suspectful of the fuel and hopefully the volley ends there.....but if it doesn't then you're going to need a higher pay grade to figure it out. Fuel pump, wiring to the fuel pump, coil or wiring to the coil? Etc....
