Running warmer than normal

The truck still runs great, but it is starting to run a little warmer than I am comfortable with. For example I was going down the interstate and I was on the throttle a pretty good bit running about 75 mph and I looked down and my temperature gauge was 3/4 of the way up the gauge. My truck never runs past 210. I kept an eye on it and when I slowed down the gauge fell a little bit. It did not have a gradual fall though which threw me off. It was more of a sudden drop in temperature. Any ideas what it could be? I just replaced the thermostat and ots still doing the same thing.