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Discussion Starter #1 (Edited)
1994 GT 5.0 t5
Optima Red top battery, PA Performance 200 Amp alternator

So Ive been dealing with this odd voltage drop/surge for the past few weeks.
Starting and idling is normal, can turn on running lights on normally. Once I turn my headlights on or activate flash-to-pass, there's a voltage drop around 4-5 volts and RPMs surge around 200-300. Hasn't caused the car to shut off yet though.

Once this problem occurs, I cannot replicate it for a few hours.
[turn on headlights, volt drop. Turn lights off and then back on, no problem. 2-3hrs later, volt drop.]
Doesn't occur when using turn signals, hazards, or any wiper/washer mode. And doesn't matter whether accessories are on or off.

Main headlight switch (BWD) and wiring harness (BWD)
Multifunction switch (Motorcraft) and harness (OEM)
Replaced fuses and relays (to be on safe side)

Cleaned all the connections and upgraded all grounds and added some with 4AWG fine strand speaker wire.
Brand new battery with new terminals and cables. PA recommends using a #4 gauge power wire, but I used #2 gauge for insurance.

What am I missing?
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