I have had a 99 cummins for 6 months now. It has 175000 miles and ive had a few computer problems but have it runnning fine now. I haven't towed anything with it until this weekend when i pulled my new fifth wheel toy hauler about 150 miles. The temp climbs when doing hills. The check guage light came on once. I was taking it easy and babying it. It has more power but the temp will just come up. Any suggestions? It has good coolant. Right level. If it wasn't for this problem i could have kept up with my dad's 07.5 duramax.