I drove my '96 Mighty Max up to Ohio last summer in 103 degree weather. When I was driving home I got stuck in a 20 mile long traffic jam, sat in traffic for 8 hours with 100+ temp outside. I saw a ton of newer and older vehicles stalled on the side of the road overheated. The local Rescue Squad was delivering bottled water to people that were stranded. My temp gauge never came past the 1/4 mark and I maybe turned it off twice. I had a smile as big as Texas on my face because I thought I had a freak of mechanical engineering. A few months later when colder weather started coming around I noticed the heat would only be lukewarm. I decided to change the thermostat and found it stuck open. Changed it, the temp gauge sits right at the half way mark, and heat will burn you out of the truck. I would definitely change your thermostat.