Sounds like it is leaning out as the tank level drops.  Using a uniflow tank set up could help here.  You use two clunks, use your  muffler vent line for fill vent only, plug the line before flight, do not attach to the muffler nipple.  Use a clunk on another line and make it about a half inch shorter, some tie it to the pick up line, I do not think that is required.  This works because the head on the muffler pressure is the same both upright and inverted.  With this setup the tank should be centered with the needle valve, normal setups should be about 1/4 to 1/2 inch below when upright.