My dads next door neighbour had exactly the same, and new starter, battery, alternator, still the same issues, then asked me to have a look, did the usual checks, all fine until I checked the resistance under load when it went sky high.
Anyway it was the actual connector on the end of the loom wire that goes onto the solenoid from the ignition switch, cut wire back soldered a new one on with adhesive lined heat shrink for a total cure, so worth a look, as this joint has massive resistance, but was as ford made it.
tabetha