Need help with electrical problem

My engine went from running well to progressiveIy running worse and now not running. I have a Pertronix ignitor ignition system. I am getting intermittent spark at the plugs when I ground them. With the key on in "run" and grounding a plug on the exhaust manifold I get an intermittent spark. But when I pull the distributor coil wire and ground it I get a consistent strong spark. Practically new plugs and wires and cap. Changed rotor but no change. Plugs are not oil or gas fouled. I replaced my firewall to block ground and put a new 4 gauge ground wire from the block to the frame. Any ideas why I'm getting intermittent spark when I am getting strong spark at the coil? Thanks for any help.