Simon,
Try the following;
Sediment bowl, have you dropped it off to check it isn't full of algae etc?
Turbo to intercooler hoses for delaminating/ leaking?
Fuel system air leaks?
Poor lift pump operation?
Pressure line from turbo to top of injector pump? This is for boost enrichment.
The following are not common, but will give similar symptoms;
Seizing alternator.(I have seen one actually STOP a 300Tdi

)
Sticking governor pin in injector pump, the one that runs up against the inverted cone in the pump top housing(Aneroid/ boost compensator) when this sticks IN, it will fail to enrich under boost, producing low boost and no power. (Only seen this ONCE)
Fuel tank pick up intermittentlt blocking due to foreign body in tank, OR split so that air can get into the fuel system everynow and then.
Also, have you jacked it up and felt for binding/ disintegrating wheel bearings?
Don't worry Simon, we'll find it!
JC
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