Engine hesitates and is jerky at around 1500-2000 revs?

Hi. I bought a 1998 Rover 420 gsdi turbo diesel car.Although runs OK the engine hesitates and is jerky aroung 1500-2000 revs.This happens when the engine is cold or hot,going uphill or downhll and never does this either under 1500 revs or over 2000 revs.I have filled the tank so fuel isn't the issue,cleaned and checked the air filter housing and am sure the head gasket is not leaking.Can someone give me any advice?? Cheers Sean