Another loss of power

Ok I have scanned the threads on loss of power, but can't find one that seems to replicate my fault.
05 plate TD4
Will drive perfectly for about 20 min, then the power drops and it doesn't matter what gear you are in she is labouring. Stop, turn off ignition, turn her back on again and she is right as rain.
No black smoke, no whistle from Turbo.
Thought it could be pump related, but I would have thought that would happen all the time, the same with the Turbo.

Any ideas?

OK - had the Landy put on Test book this morning, all values seem to be in their correct limits, so we took her for a drive until she lost power and went into limp mode, still no errors on the Test Book and the values foe AF etc. were within limits - confused!!

Anyway, taking off the oil cap, seemed to be a lot of pressure building up in the head, so we replaced the Crank Case Breather filter (it looked pretty oily and clogged) - so the Jury's out at the moment, I will see how she performs on the way home.

Another update...
Fuel filters replaced and pipes cleaned, but still when cruising it will loose power and pressing the accelerator does nothing - then all of a sudden it will pick up again....

The garage thinks it now could be a collapsed CAT, but I am not sure - the jury is out...

Have you replaced the MAF? Exactly the symptoms my Passat gave when the MAF died.

Disconnect the MAF and run it to see if it failed

Please keep us updated. I'm having this exact prob.

Sounds very similar to the issue i had with my 320d BMW, it turned out to be the MAP (not MAF) sensor