I bought this 300 tdi as a non runner with head gasket failure. I had the head skimmed and new gaskets/water pump/p gasket/ header tank/temp sensor/radiator.
Got it running and found that it wasn't as powerful as I'd hoped. I changed the timing belt and found that the timing was out so that improved it a bit. Not enough so I put on a larger intercooler and did the injector pump mod alignment. This did improve but suddenly the engine went lumpy. I quickly found it was the injector seals... either I'd damaged them putting them in or something but 3 needed changing... so I went and got a new set of 4 (the timing belt kit came with the car and had new seals with it... so given I don't know the history of the kit I might assume that they were damaged in the packaging... clearly not me fitting them... meh....
All this improved it but it still took a lot to start it... had done since the head gasket was done.... now new problem it cuts out when the revs drop... bit of checking and diagnosed the lift pump had failed... replaced and all good...not driven yet but started perfectly and picked up revs nicely ... but suddenly it picked up revs on its own to 2000 and sat there... won't drop down.

Little stumped... I don't think it's the fuel pump mod as surely this only starts to take affect when you move the accelerator pedal.... This did it on its own????

Any thoughts?

Bit silly, but check the accelerator cable, the first big adjuster can come undone, the adjuster unit 'falls out' of the holder and revs rise ... or don't go high enough when you want them too ...

Yeah. That had occurred to me. So I checked that already. Thanks tho.
Think I'm gonna have to set the injector pump up again and see. Make sure there are no loose adjusters etc.