Sounds like my GMS 47's that I never figured out.

Things to try. An OS #8 glow plug, an OS #6 is recommended. Check all screws on the engine making sure they are tight. Is the clunk in the tank free and rattles when the tank is empty and the plane is held nose up vertical? It could be bent forward in the tank. Do the fuel lines fit tight, or are cracked? If so replace. You might want to just rebuild the tank, could be crap in the tank. They take a lot of abuse being pulled on and off each flight. Check the engine mounting bolts, loose engines run lousy. Also you might want to pull up the the exact instructions for that engine and reset the idle needle valve to the default setting and start all over. Just some thoughts.

http://manuals.hobbico.com/osm/46ax2-manual.pdf