stevefly i had the same problems with my 2003 td4, i changed fuel filter, fuel pump, maf, boost pressure switch, crankcase filter, all the vacuum hoses. the turbo and boost pressure solenoid was replaced 2 months before these. but still no joy. i was telling a colleague about this who introduced me to a mobile diesel mechanic, he came and plugged in a tester rattled a couple of wires and said yep its your engines wiring harness, changed the harness cleaned out the high pressure fuel sensor and its been perfect since. i am not say that this is what's up with yours, what i am saying is these are a mind field and can end up costing big bucks so for the sake of £80 or so let the man with the right diagnostics have a look first.