griesel, You may be having a problem known as "Fuel Foaming"

When the vibration conditions are right, fuel can turn (temporarily) to foam.

If you have another tank, try hooking it up to the engine, but leave it outside the plane and see if the engine works as it should (You can even use your fuel jug for this test)

If it works, try padding your plane's tank with foam rubber to help keep the vibrations down.