I know naff-all about diesels .. but some google searching has found this :
on about diesel hummers ....
We talked to a tech at Stanadyne. He said too many tech's change injector pumps when the problem is really the injector itself. If the injector itself is bad it won't always show up in the ODB II codes. The tech recommended we put the motor back together and find 4 injectors off of a truck that is running Ok and swap out one bank at a time to see if this fixes the "running rough/misfire" problem before tearing into the motor further.
Got the truck together last night; except for setting the timing more accurately, seems to run quite well with 4 new injectors in, rather than putting in an injector pump and who knows what else. Guess it was the injectors all along
and on a golf :
Possible Symptoms
* Loss of Power at high Engine Speeds
* Engine difficult to Start
Possible Causes
* Start of Injection Timing Regulation not OK
* Injection Pump Control Range blocked/dirty (e.g. by Metal Particles)
* Engine has stalled / ran out of Fuel recently
* Wiring/Connectors from/to Injection Valve (N108) faulty
Possible Solutions
* Check/Adjust Injection Start
* Check Fuel Supply
* Check/Replace Fuse(s)
* Check/Repair Wiring/Connectors from/to Injection Valve (N108)
another site .. not sure which marque .. talks about "pump advance/retard stuck in a fixed position"
probably you have already considered all these points .. just trying to throw ideas in ..
good luck