Hello Paul.
When you said the coil wiring had been tested, how exactly did they test this?
The problem isn’t the coils; I would start by checking voltage at the main coil feed and checking for voltage drop under load.
This test can be preformed easy on a Dyno by watching voltage drop under boost, if voltage drop's erratically under load, then a relay is required to the battery, well in your son's case, R33, hook up to main power feed in fuse panel under the bonnet.
Also inspect the coil plugs, with heat and ages they become brittle and crack. One thing we find is due to the age the vehicle; the wire gets "what we call" green cancer and this causes resistance in the wire.
Rick
EFI Performance