big island 81

91 v6 extra cab 5 speed. got the truck with blown head gaskets from my father in law. fixed the head gaskets water pump timing belt knock sensor harness timing belt idler roller all the other belts. truck was running good but every now and then the truck would run like the timming was off then run fine again. now it revs like the timming is off and under a load wants to die has no power. holding the rpms at 3000 steady it drops to 500 rpms then back up to 3000. just had the fuel injectors cleaned by napa, i cleaned the plenum replaced all vacuum lines, ecm ,plugs cap wires rotor,mass air flow sensor/meter, fuel filter and in the process of changing fuel pump. any advice would be greatly appreciated.
MAHALO