How Can I Repair My 1998 Isuzu 4 cylinder Turbo Diesel?

What repairs need to be made, engine starts fine I start down the highway the gears shifting fine with the auto trans... then between 30mph - 40mph it flat lines, boggs down as if to much fuel is present or lack of fuel is being delivered to the engine, then the engine stalls. It seems as if the power train goes on vacation. I changed the filters gas, trans & oil / oil filter, added diesel fuel lub & injector cleaners. What the bad news?