Why is my truck backfiring and missing?

I have a 1995 Chevy truck with a 5.7L throttle body. I am having issues with it; I start it and it runs fine I even drive it and it drives fine, but only for the first 5 miles. after that it starts sputtering and backfiring. I have put in new fuel pump, new coil, i thought of doing the rotor button and distributor but i feel as though if those were the issue i would have the issues right from the beginning of starting my truck. Please help me. Thanks