Over the many years I had my previous TDi (a 90bhp 1.9), I had two separate loss of power issues; the first was due to the air hose partly coming loose, causing a loss of power, but particularly at higher revs.I do remember having a similar problem to you, struggling to maintain speed on the motorway. This cost nothing to fix as it was just one of the jubilee type spring clips that had come adrift.
Years later, I had another loss of power which took ages to diagnose, but was tracked down to a blocked oxidation catalyst, which was replaced.
Oddly, although the MAF which is apparently a common cause of these issues, it never gave me problems in all the years I had the car.
On my current TDi, I do try to keep the revs up the engine when warm from time to time which should hopefully stop the oxidation catalyst from getting blocked up; thankfully it doesn't have a particulate filter!
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