Ignition timing? OK when cold, but not after running for a while. What happens if, from cold, you start the engine, then immediately stop it again. Does it re-start then, before the engine has had a chance to warm up? Is there a crack or something loose in the dizzy cap? Or is the coil starting to pack up? Sounds like you may have two problems at the same time, not surprising if *everything* has been disturbed at the same time. I agree with Rusty's Uncle that you should re-check all the major earthing points. It sounds electrical rather than air-related.