89' Suburban runs real lean after warm

1989 suburban 4x4....5.7 LO5.....137,XXX miles.

well about a month ago it started to slowly lose power under acceleration. now it runs like a champ for about 5 minutes or so then leans out real bad. i have replaced the ignition module, pickup coil, 02 sensor, injectors, regulator and did a complete tune up . only thing i havnt done so far was drop the tank and replace the fuel pump. the strange thing though is it'll run strong for a few minutes then lean out untill eventually the engine stalls, fires right back up and runs good for another few minutes. i did notice that every once in a while the headlights will go dim for no reason and the engine will hesitate for a second while this happens so i am thinking maybe the voltage to the fuel pump is dropping off and on. or maybe im way off but any suggestions would be very much appreciated. oh and i ditched the cat a long time ago so that isnt an issue. thanks