isuzu tf 2002 with low power

hi all,
i have an isuzu tf 4x4 double cab on an 02 plate

i have a problem with low power while driving.
i am struggling to get above 30-40mph
i struggle to get up any slight incline.
the vehicle has a flat spot... dangerous on roundabouts etc..

i have replaced the diesel fuel filter, changed the air flow meter as sugested by isuzu..no joy
i have put in a diesel cleaner which is supposed to clean the fuel lines, cat, filter etc..
having spent 300 pounds already, i am seeking any advice on what my problem is likely to be and what part to be replaced..

many thanks
kevin:crying: