Hi all, any help with this one greatly appreciated.
Symptoms:
Occasional bouts of misfiring, typically when throttle is just open at 30 to 40mph.
Engine idling normally at 600rpm, revs drop to 250 then back to 600. Repeats at random every few seconds. Sometimes engine will stop.
If this happens no faults logged at the time BUT restricted performance may appear at the next start (weird) with P0088 High Pressure Rail Too High fault, clears every time with Gap IID.
Never does this when cold, only when hot.
It may run trouble free for 2 to 3 weeks then gremlin returns.
Engine runs fine under high load so I don’t think it is a fuel blockage.
Works done:
Crankshaft position sensor changed
Turbos changed
Electrical connections in engine bay checked
High pressure rail wiring mod done
High pressure rail sensor changed
High pressure rail control valve changed
All filters including fuel filter changed twice.
I’m in Tavistock Devon this week and Oxford next week, I’d be happy for someone to take a fresh look (so far three good indies have not tracked it down due to its intermittent appearance). Home is Beds/Northants border, any recommendations for someone knowledgable with these engines?
2011 Overfinch Autobiography TDV8 4.4 with 210k. Love it and it’s the only problem it has.

Notwithstanding that the crankshaft position sensor has been changed, when the idle issue returns monitor RPM with a diagnostic tool looking for spates of zero RPM. If seen then either another dud sensor or a sensor wiring issue. Otherwise I suspect the HPFP but that's too expensive to replace on a guess.

Thank you Graeme. Do you know what it is with the HPFP that would cause the rail to go high pressure? I recall reading of someone else who was getting the same high pressure fault and changing tbe HPFP solved the problem (I can’t say I understand why, I thought a failing HPFP would cause low pressure).

I don't know the ins and out of the HPFP but the pump would have to reduce its output when the pressure gets to its high limit, but perhaps it can't because of a mechanical issue. It would be good to check the actual pressure if the operating range is known but that would require appropriate fittings and pressure gauge. However I can't imagine why excessive pressure would cause loss of idle revs.

To anyone reading this thread it’s taken an eternity to find the issue but it is finally solved.
The issue appeared soon after i bought the car so first thing I had done was to get the high pressure fuel rail sensor changed by an Indy I have used for years and who are normally very competent. This time they fitted the wrong sensor. The correct sensor for the late L322 / 405 4.4TDV8 is Bosch0281008513.
Bosch 0281006064 is often marketed as a suitable sensor - it isn’t!
Many thanks indeed to Maltings 4x4 in Doncaster for talking me back through the basics and got to the root issue in one phone call. I wish they would open a branch in Bedfordshire or Northamptonshire for servicing, they would have my custom!

Good news and thanks for posting the resolution.

Out of interest, did it always start OK? 2012 Vogue TDV8 in Aintree green over sand

It has always started fine and behaved perfectly when cold. The only time it has not started was when it was hot and in heavy slow moving traffic on the M5, it stalled on me and I managed ti coast onto the hard shoulder. It would not restart. I waited for it to cool for half an hour and it started again. It was a sod to drive for the next hour.