Looking for some help with our 4010 gas.. it will run all day long at any rpm if it is sitting still or driving down the road. When we put any kind of load on the engine it will begin to "loap". Example- pulling a 150 gallon 3pt mount sprayer at PTO speed (2000 rpm) it would drop to 14-1500 then back to 2000 over and over again. We have cleaned the carb, replaced all the plugs and wires, new electronic ignition, coil. Removed the mechanical fuel pump and replaced with electronic. This has been a problem with this tractor for a long time, we normally just do light duty work with it, however farm is growing and we really need it to pull. What do we try?