Isuzu tf low power

hi all,
i have an isuzu tf 4x4 double cab pick up.
i am having serious problem with lack of power.
i have had fuel filter changed and air filter.
i have had new air flow meter fitted but still have lack of power .
its like a total flat spot.
i cant travel up hill etc, or pull away safely due to no pulling power.
there are no fault codes, and i have put a diesel cleaner in to clean the fuel lines etc.
can any one help ? any suggestions ?
many thanks kevin

I doubt you'll find an answer here. This section of the forum is for Rodeo/Passport. They never came with diesels.