Echoman, I don't think what is happening is vapor lock like in carburated engines of long ago. If you were to get out of a vapor locked vehicle, raise the hood and look under the air cleaner, you would see ice build-up on the carb body, that was my interpretation of vapor lock, soon as the ice melted, the carb would work again. The Echos I ran into this problem on, in trees just like you, I tried all kinds of fixes, and it turned out that all I had to do was open the gas cap and it would solve the problem for 15 to 20 minutes, then I'd have to do it again, so I knew it was a fuel vent problem, I can't recall right off what I did to fix it permanetly but try the fuel cap thing next time it happens and see if that solves it and maybe by then I'll remember what I did. These were new Echos 3200/ 3400's etc. and I remember a lot of different guys were bringing 'em to me for this so it wasn't just one crew, it was a saw problem, not operator error.