If the engine runs better when the plug is being heated that means one thing, the idle is too rich. When the mixture is too rich the plug element gets cooled by the overly juicy mix and the rpm's drop giving you the symptoms you have. That could be because of a badly located fuel tank or any number of things already mentioned but why don't you just try leaning out the idle screw. If it is a true air bleed carb turning the needle counter clockwise will lean it out. Try a quarter turn at a time untill you get it close, then fine tune it. When it is right there should be no difference between having the glow plug hooked up or not.

Good Luck