I've had the same problem for several years. One time, cleaning the carbon deposits from the cylinder head helped the problem...for a few months. The next time I took it to a repair shop, who re-built the carb for $400! It fixed it for a season. The next time I bought a rebuilt carb for $99. This, again, worked for a season. It is starting to do it again this spring :(
It must be something fuel-related, but not sure what. The $99 rebuilt carb almost seems like the easiest "fix"
Jon
June 2009