As loads increase, more spark voltage is needed to fire under the increasing cylinder pressure. If this is your problem, check the ignition components (plugs, wires, cap, rotor, coil). Badly performing components can block and/or bleed off the needed voltage to fire under the higher pressures. Fouled plugs are common (at least in mine with poor valve seals). Wires get old and die, even some new ones of poor quality have caused problems. Clean/scrap the cap terminals both rotor side and wire side to shiny metal, check for carbon tracking or cracks (same thing for the rotor). Never had a coil problem, maybe swap in a spare.