Also check the voltage to the (+) coil wire. I know the stock '68 wires (and maybe other years) were a resistance wire meaning they only had 9 volts to the coil when running. The HEI's and MSD's require a full 12V or strange things happen. You can try a jumper with a wire direct from the battery to the coil but remember you won't be able to shut the engine off with the key and you'll have to pull the wire off to shut it down. Set it up with a clip or a way to pull it off. Also don't leave it hooked up very long without the engine running it can be hard on the coil.