Not enought air flow or, snd I know that this is way out there, but I have seen it for myself. The crank shaft slot key,(wooddruf) has a little wear causing the engine to fire a hair early or a bit late which could cause hard starts, not starting at all or slight to just plain overheating due to the coil generating it's power at the wrong time. If I had not seen it for myself I would not have belived it. I still find it hard to believe but replacing that little piece if half moon metal took care of the problem. Look at the key real close, if it is slightly indented at all replace it.