First thing I would swap would be the MSD box.
One of our cars back in the day lost a tenth over time, car was running low tens. Forgot what led us to it but replacing the box brought it to life.
Two converters - same result, eliminates that I think.
Had a bad digital 6 box years ago that slowed my car down 1/2 second, but not on every pass. Maybe once every three or four passes.
Set up to run 10 flat, would run 10.50ish when it occurred.
Chased everything most of a season before swapping the box out and finally fixing it.
Good thought, could well be the box.. capable of wierd issues
