Have you bothered to test the glow plugs to see if they even need replacing, or are we at the point we're going to throw parts at it?
Diagnose, verify, repair...
FWIW, I have a few FICM's on my shelf that read a perfect 48+V at all conditions, but the truck runs like absolute garbage.
To me it sounds like your FICM is having issues, if the voltage is jumping around like that.