My truck seems to be missing when I get on it kind of hard, I just noticed it yesterday. when I hit the throttle (about 1/2 way) it seems sluggish and misses. The truck was running at about 190 degrees, and it was about 60 degrees outside. It has 40,000 miles on it, but about 30,000 of those has been pulling a 14,000 lb. trailer around. Any suggestions on what it could be?