I reread your post and see that you are runnimg the MSD. With most MSDs you should not have a ballast resistor.

Where are you powering your MSD from?

I would still do what I suggested in my post above, but to the MSD power wire.

I do suspect that the way you have the box powered you are losing voltage to the MSD because of a bad connection in the bulkhead. I suspect that the MSD power is being taken off of the original ballast wiring.

PM me if you would like some additional guidance.....