Spitfire Intermittently Stops Firing

Recently I believe my and my father's '76 Spitfire has been intermittently stopping firing, and we haven't been able to figure out why.

At first it would be very brief, almost like a hiccup. I would lose all power and my revs would drop to zero for less than a second. But the more I drove it, the longer the engine would cut out. One time it cut out for about 2 full seconds and then backfired so loud I thought the engine exploded.

Has anyone had any experience with this issue or have any ideas on how to fix it?

We've put in an all new wiring harness, the last owner put in a new spark distributer, and all of the spark plugs are clean and adjusted properly. I'm not sure what could possibly be going wrong electrically for this to be happening.