Intermittant Starting Problem

My car is a 72 Dart Swinger with a 69 340. The problem I am having is that about 75% of the time the engine will not start from the key, however, it will start if I jump the solenoid. I have rplaced the ballast resistor and this still doesn't help. The problem seems to be from the key to the solenoid.

:clock:ANY HELP WOULD BE APPRECIATED