Big,
Check the spark plugs first like Bryan suggested. If you have Champion plugs, for some reason, "they just don't make em' the way they used to". Quite a few folks have been having troubles with the Champions and "under load" missing is one common problem. Also, If you're using radio resistance wires, you do need to install a set of solid core wires. This delivers maximum power (spark) to the plugs which is needed especially for the tractor in the "load" or working mode. If you can't find the correct wires I make them for Allis engines. Steve