Engine miss under load - ignition concerns

Over 3V at coil NEG sounds high to me

Coil+ voltage is going to vary

13.65 to ballast should be fine.

I would REALLY check ECU grounding, and body to engine ground

Don't discount "simple stuff" like cap, rotor and coil wire

I'm not a chemist. Don't discount plugs. They seem to get the ceramic "cooked" under certain engine conditions. Years ago I fought a problem I "outthunk" had CD ignition, and a high speed miss. Plugs "looked" fine. FINALLY changed the plugs, and WHAM Ran great