Here's a WAG. Perhaps the pick up body is not functioning properly. As with my Echo, this part in the fuel tank appears to act both as a fuel filter and to move around in the tank, i.e., drop to the bottom, allowing fuel to flow irrespective of the position of the tank. If the part is partially clogged, and turned to a particular position, the fuel flow might be stopped. Next, if the pick up body movement in the tank is restricted -- and does not fall to the bottom (irrespective of position of the tank) fuel flow may stop. The Stihl instructions say to change this part yearly. As to which of the guesses is better, the latter may be more correct since this has been occurring since the beginning.

Replace the fuel filter and squeeze the fuel lines between your fingers checking for cracks, holes or leaks. Ethanol in our gas is hard on these lines and gaskets. Next, run a tag wire up through the gas cap and clean out the vent. If this doesnt work its time to replace the carb diaphragm and raise the metering lever a little. All standard stuff. If you need help setting the carb up after you have checked the lines and vent, all you have to do is ask.

Best wishes,
Bob