1989 GMC k1500 225,000 miles 5.7

My truck has been idling poorly for the past few weeks, for example after the truck warms up it idles so slowly that it dies. The truck always starts back up right away. I replaced the idle control motor but that did not fix the problem. I performed a tune up (fuel filter, wires, plugs, rotor, cap) and after about 200 miles of smooth running the engine misfires and dies while driving although it still starts up right away. The engine sounds as if it is not firing on all cylinders.