However, it it runs pretty good some of the time, it is likely not timing, unless, say, the advance mechanism in the dist. is rusted up and for some reason sticking when it gets warm. Remove the dist cap and twist and release the rotor CW. It should have spring pressure, when released should snap back to original position, and move freely.
Is it possible you are experiencing fuel boil/ vapor lock? Is any of the fuel system close to exhaust, or moved from stock location? Is it possible the tank is not venting or that the tank pickup screen ("sock") is plugged
