I don’t think it is the spark plugs, I do thing that you may need to replace your air filter. When you first start out your air pump may be still on and the throttle plate may still be opened up still. As the care gets warmer the air pump gets turned off and the throttle plate closes a little.
I would also flush out your radiator and also add some red line water wetter. It will help keep your truck cooler. I would also check your cats out to see if they are plugged. You can kick them to hear if they rattle. If so you me need to replace them.
For your air filter I would recommend a K&N air filter.