Chevy Blazer

I have a 1995 chevy s 10 blazer, v6 vortex. At times it will start running rough and when I go to put on the brakes they will go to the floor. I can then take it on a dirt road and slam on the brakes and then the brakes will work and the car will run fine for awhile again then it will happen all over. I relpaced fuel pump and the sending unit, the rollover valve, the EGR valve, replaced sparkplugs and wires, the distributor, the map sensor, the throttle position sensor, and one oxygen sensor and still can't figure out what is wrong. Every time I have the codes checked they say the same thing, map sensor EGR, throttle position sensor and o2 sensor even after replacement. I also put new fuel lines under the plenum to the injectors. Any help would be wondeful