Its a strange one then, have you checked the breather hose on the fuel cap, try starting it with the cap off!
Fuel filter on the petrol cap clean and fuel runs out at a decent rate? Check the pilot jet AGAIN
Clean the earth on the CDi unit?
Recently had a chain saw that would run for 10-15 mins or so then refuse to start for a while it was the coil breaking down.
As you say when its running you cant fault it probably is simple when you find it