Engine dies while driving - tach drops to zero (MSD)

[AdamIsAdam]

My FFR Cobra just developed a problem. I can be driving, idling, anything, then all of a sudden, the tach drops to 0 and the engine stalls as if someone turned the key off. If I'm moving, I can turn the key off and then on again and let the clutch out, or just restart it, and it's fine. I was thinking the ignition switch might be going bad, but I do not lose any other electrical at all (gauges, lights turn signals all remain on). Plus, today I was driving and decided to turn the key off while in gear and the tach does NOT drop to zero when I do that; then I turned the key back on and it continued running of course. The problem is happening more frequently now too.

I've checked the spark with the paper clip trick and it's fine, which MSD says that means the box is fine. I've checked the ohm reading on the magnetic pickup and it's within their specs. All grounds appear to be ok. I'm at a loss as to what to look for.

The car is a new-to-me Factory Five Cobra that only has 1,600 miles on it, but is 6 years old. It's a 347cid with a holley DP and mechanical fuel pump.

IGNITION SYSTEM:

MSD Digital 6AL box, MSD 8598 distributor. Coil blaster 2. I actually replaced it last month because the coil died on me. Totally different symptoms: car stumbled and died when I first started it and wouldn't restart. Coil did not have continuity. New coil fixed it right up.

Cap, rotor and plugs are new. Car runs great when it's running.

The tach is from Speedhut.

I hate to point to the little red box, so please advise on how to troubleshoot. As I said, the entire car looks brand new, so no rusty ground bolts, and every ground wire I can see is really well secured.

Thoughts and suggestions welcome!