Engine misfire under load

What is your question?

My 96 Chevy 1500 pickup shows code PO300, (multiple cylinder misfire). This occurs when the engine is put under a slight load, gentle acceleration, in fourth gear or ovedrive as some call it. It's pretty severe, but if I cause it to down shift or simply accelerate more it will go through the misfire. I hadn't tuned the truck up in about 85000 miles, so, I replaced the plugs, plug wires, distributor cap and rotor. No fix. I replaced the ignition control module. No fix. I replaced the throttle position sensor. No fix. I replaced the ignition coil. No fix. I cleaned the MAP sensor. No fix.