if you have the old ballast resistor wire connected together..then the msd is getting both run and start voltage unless one of those wire is having problems from the ignition switch..or firewall bulkhead...
check the voltage if you can of those two wires before they are jointed...one at start..one at run...
but you might getting one or the other back feeding the other...
another simple thing...use a jumper wire from the battery straight to the msd switch wire....the one coming from the ballast resistor to see if it is not the box or it is wiring...
