I have a 1990 f85 2.0 ltr petrol fourtrak with running probs. it starts fine from cold and will sit there ticking over for ages, but if i drive it for 3 or 4 miles it loses all power, coughs, spits and backfires. The points, dizzy cap and rotor arm have been changed. I have been told it could be the coil overheating and to check the ballast resistor. Does anyone know the voltages of the ballast resistor so i can check it is working correctly.