I'd try another spark plug first. It may sound funny, but sometimes a plug will seem to fire okay out of the engine, and not fire well when it is in the engine, and under compression. BTDT.
When one of those engines that has been running starts to act up, check the plug, clean the points in the magneto, check all of the electrical connections, and make sure that the check valve in the fuel line isn't stuck. That will usually get you going.