Sponsored Links (Register now to hide all advertisements)
Check the easy things first, is the best course. If you check the wire from the ballast resistor as meric42 mentioned, it should have a steady voltage. The ballast will drop the voltage lower than 6-volts but it should be steady. If it has steady voltage then check the coil primary winding. It should be 1.5 Ohm for a stock 6-volt system. If it's OK then replace the condenser & see if it runs.
The only way to check a coil is to heat it up with an actual coil test set but most folks don't have that luxury. If it is still intermittent then look for the best quality coil you can get. Bosch silver coils are epoxy filled and can take heat. Some say the Flame Throwers work OK but I haven't used them. I have to assume that your Mallory distributor uses a remote mounted can type coil. If I'm wrong then let us know what it has.