Now power

I have a Mazda b2600 1989 we have replaced the fuel pump, ox sensor new spark plugs air flow, done just about every thing new in the fuel department. Now we still have the problem that when it goes up hills it looses power and acts like it is not getting enough gas, sometimes backfires going down hills it runs fine on flat ground and we have had the injectors cleaned also. Being we live in Costa Rica we have lots of hills