FB,
There's a few things i would think of:

1) check for an out-of-round crank that is causing vibration which is foaming the fuel;
2) check the tuning of the engine. If it is getting hot enough, it will vaporize fuel in the line and may look like foaming fuel.
3) Look for a problem in the engine mounting, including a cracked crankcase that is allowing the vibration.

If you're not able to find the problem there, send it for service.