2003 td4 Freelander

when the engine (130,000miles) is warm there is a slight missfire about 2000-2500rpm before and afetr this rev range it is fine, it is also more noticable under load, towing, for example. This does not happen when it is cold/just started first thing. The fuel pump is ok and the fuel and air filters have beem changed, it has been checked on a computor with no fault showing, any suggestions wouls be really usefull. Thanks

i would start with the cranckshaft breather and maf