Engine sometimes lumpy

HI all,

I have a X reg Fabia 1.9 TDi, 145,000 miles. Recently it has been lumpy occasionally at 2,200 rpm in 5th gear (approx 70 m.p.h). If I increase revs a bit then it is ok. Performance generally is great, loads of power at high revs etc.

This is intermittent but is increasing in frequency and getting slightly worse.

The car was serviced by a Skoda dealer recently and also had the timing belt changed. (It was lumpy on occasion before this too).

Does anybody have any ideas what might be the cause and what order I should look at checking things?

Many thanks