The PD130 motor should normally do its best work between around 1,750 and 3,000rpm.

It sounds like the camshaft timing is slightly out following a cambelt change. On this engine, the diesel injectors work like hypodermic needles, they are pressurised by a lobe on the camshaft which pushes the 'plunger' to atomise the fuel. If the camshaft timing (which is adjustable) is slightly out, pressure will build up either too early, or too late.

This article describes exactly how the camshaft timing works on the PD engine, and how altering it affects driving behaviour: www.autospeed.com/cms/A_112521/article.html

Any VW diesel specialist should be able to set it up correctly for you.