Hi Tom,

Odd how it misses more after motor mount replacement. Check grounds.
Take apart the nest of wires at the battery positive terminal, shine up
and use conductive grease or Penetrox-A.
Maybe a loose ECU signal ground wire (2) at the intake manifold?

You might look at the plugs carefully to see if there's any clue for which
cylinder /s is missing. Check the plug gap.
You can do a compression test while the plugs are out.

Since you inherited this problem, I'd service all of the ignition connectors.
Clean with isopropyl 90% and a soft toothbrush, blow out with air and inspect the
female connector pins, maybe you have some damaged/spread pins?
Adjust or replace any bad pins. Get connector parts at www.davebarton.com
While you're at it, look at the AMM, and CPS connectors.
Use Penetrox-A on the pins. Corrosion and / or loose connector pins
for fussy shielded wire engine sensors invite trouble.

Good luck, Bill