I've got a 90' RC 4x4 auto with a 5.9. I noticed the other weekend when I took it out on the interstate that the she started to run warmer then normal. When I got her home I threw in a 180 t-stat and did a full flush on the system. Took her back out and it did the same thing. When driving in town she runs right where it should. The only difference in town and out is that I use overdrive when on the interstate. The only other thing I can think of is that my water pump is getting weak. ANy ideas? Thanks!