Td5 Miss fire/Lag Help

Hi. Hoping someone on here can help. Ive got a 2000 defender td5. And it’s developed a miss fire/lag at high revs in all the gears. It climbs up to high revs and just drops all of a sudden even with my foot flat to the floor and starts dropping speed unless I change up a gear. I know they arnt made to go fast but gets to 65 in 5th gear and again hits that limit and just dies on speed. It doesn’t do it when I’m parked up and Rev it. It revs all the way with no miss fire stationary. There is some oil present in the red Ecu plug but not in the plug on the injector harness.
MAF has been changed. Injector harness changed. EgR blanked off. Fuel filter changed.
The fuel pump is making a whining noise but has done for ages and there is a possibility that the fuel pressure regulator is leaking.
Any help or advice will be appreciated. Thanks!!

Get the oil out of the harness first, or change harness. If it still does it check what boost it’s making as it sounds like overboost symptoms, ecu then cuts fuelling causing ‘misfire’