First few seconds huh? That’s quick enough to still be in the mid leveling zone id think.
Do you have job prints? Other than the Regulator Board, maybe look at the modules in the down direction high speed circuit.
Do you have a second unit to swap tach with?
No RVU to trap a fault?
Hopefully you’ve got the set up manual for the regulator.

IIRC, 58, 94 almost always show on any fault condition.