Check the resistance of the primary circuit. Points operate best at 4-5 amperes. For a running tractor with alternator/generator that's 14v/5 amps max or 2.8 ohms. If the coil and ballast resistor aren't giving you that much resistance, increase the resistance of the ballast till you get it.
Otherwise a bad condenser will do it. The condenser is there to absorb the high voltage of the arc when the points open.....voltage generated by the coil when the points open and the current is interrupted. The nature of the coil is that it attempts to keep the current flowing by generating whatever voltage is necessary....within it's limits, hence a high voltage arc across the points.
There is a voltage where, if high enough, will cause excessive pitting. The idea is to condense, or absorb that initial spike of voltage in the condenser and then give the energy back to the circuit at a lower voltage after the initial spike.