I'd suspect its the Ignition switch or the MSD box. My trust level in the 6AL boxes is pretty low. I switched to a MSD Street Fire 5520 after the last failure. It has the same options as the 6AL but not the same MJ amperage output. $100 cheaper. Meets my needs fine.
You don't have a heavy key ring per chance?
I've had pushed back pins (not locked) on the Green/Purple Dizzy connector at the MSD dizzy giving intermit connections. A wiggle check with the engine running should work.
Below is the FFR wiring diagrams.
https://www.factoryfive.com/wp-conte...ness-rev-T.pdf
The Gray wire on the MSD boxes is your Tach feed. Since the tach dies at the same time as the engine its either the Ignition switch or the MSD box or a loose connection. You can use a piercing probe to monitor the power wire to the the MSD box.
https://documents.holley.com/6425.pdf