I would look into (2) things... CARB. TRASH, and rollers. If the rollers are too far gone (can take 1 day) they will shimmy around, and not open the Var. until they find a good spot and will then open it. This is a maybe... I would think removing/cleaning the in tank screen, replacing any in line filter, and a through carb. clean will do it. A belt is a good idea too but will give a constant problem. Not an intermittent one. Of course check the plug. Unscrew the plug cap, cut off 1/2" of conductor, and screw it back on. This will eliminate burnt wire at the end. Check the vacuum line for hardness or cracks, and check the inlet side of the carb. manifold for cracks as well. I'll eat my keyboard if it is the belt... ~Pass the salt please~