problems narvara

Hello I have a 2003 2.5 Navara twin cab it is giving me some problems for the last while it is down in power not so much when its cold but when it warms up 1st & 2nd gears are fine but when you use 3ed & 4th it is very bad no power and chucking quit a lot but not all the time but most Ive tryed changing the filters and gave it a good service but that did'ent help any ideas please
Paddy31