Performance issue

Hi folks,

I'm new here and was just wondering if anyone can help with a small issue I have with my 2007 Focus Zetec Climate TCDi.

I bought the car three months ago and have generally been very pleased.  When I bought it, it had done 131k miles and had always been regularly serviced by a Ford Main Dealer by its 2 former owners.  The last owner had it for almost 8 years.  At the time of purchase, the cambelt had just been changed.

The one problem I've noticed with it is something that I put down to poor timing originally.  At about 1800 revs - especially noticeable in 3rd gear for some reason - the performance is what I can best describe as lumpy.  It stutters in a way that, with previous petrol engines (this is my first diesel), I'd have put down to either the timing being slightly out, or dodgy plugs.  The zone is usually between 1800 and 2000 revs, after which it seems fine again.  I mentioned it to a friend, who suggested it was probably dirty fuel injectors.  He told me to try a bottle of injector cleaner with my next top-up and see if that worked.  I did, and it seemed to be alright again for a few days.  Then the problem returned, and seems to be a little worse now.

I'm guessing that it's probably pointing to worn fuel injectors.  Either that or the timing wasn't adjusted correctly after the cambelt was changed.

Like I said - it seems fine up to about 1800, and is fine again after 2000 (I don't generally drive much above that, anyway).  It seems odd that it's especially noticeable in 3rd gear, though.  If I accelerate gently, it's hardly noticeable at all - but really bad if I'm punching it a bit!

Any thoughts, please?

Thanks for any suggestions anyone can offer.