A couple weeks I was driving my truck n out of nowhere it started jumping and shaking real bad as if it was missing on multiple Cylinders. It then shut off and would not start back up (would turn over but wouldn't fire up). After sitting for awhile it will start up again but after a short while does it again. Has been doing this ever since. Will only run good about 20 mins after a cold start but with the outside temp warmin up it seems to shorten the running time. Upon further investigation came to realize what is happening is there is no spark coming to the plugs after it reaches this point. I replaced the distributor and cap ignition module, plug wires, and coil (all with junkyard parts) but continues to do the same exact thing. I don't know what else to do! What could the problem be?