[Franco Cozzo]

Check any earth points on the cylinder head given the head has come off, make sure they're clean and secure.

Also check the ignition coil given it sounds like it's breaking under load, also even though it's got a new fuel pump get it hooked up to a fuel pressure gauge and watch it while getting stuck into the car.

I had a VY Clubsport I fixed playing similar shenanigans - started and drove perfectly fine, until you got stuck into it then at 3500 RPM it just stopped and wouldn't go anymore.

Fuel pressure was 58 PSI at idle, on starting and driving normally but if you got stuck into it, dropped to 19 PSI.