dublenaught,
Excess fuel can foul plugs, but usually they will just get sooty (dry soot) and keep going. If the ignition is weak, and the amount of gas too much, the plugs can be overcome and misfire.
Much more prevalent would be oil fouling. This will result in a wet, slightly oily plug, and it doesn't take a whole lot to short the plug out.
As you increase the load on the engine, cylinder pressures increase, and it makes it more difficult for the spark to jump the gap. If your rig is only missing under a moderate load, the first thing to check would be the ignition system.
Under load, engine vacuum drops and the power valve opens to give the engine a little extra fuel. This extra fuel could be enough to complicate a weak ignition.
Find a hill nearby and drive the truck up it, trying to maintain a steady speed without downshifting or abrupt throttle changes. Note when or if it starts to miss, and if it does it consistently.
Your mechanic should be able to read the plugs and interpret what they indicate. If they actually are fouling, they will be dry & sooty, wet & sooty or just plain wet & oily.
A ignition system in good shape will fire through all but the worst conditions inside the engine. Good luck.
Stuart
