I wasn't sure what it would be but I did a quick search and identified a very probable cause.
There could be a blockage of some sort in the line from your overflow / reservoir hose. Therefore the system could actually be low but it appears fine while looking at the reservoir. With it cool, open the radiator cap and look in there. (this is what my research led me to as far as the suctioned down upper hose)
Also, when was the water pump done? Has the heat been ran since it was put in? Maybe air in the system somewhere.
My mom's old crappy 95 Contour would do similar things. Driven around town it was fine but as soon as I hit the freeway with it, it would start to overheat. It also had a faulty cap that wouldn't hold pressure like it should and the coolant would slowly escape as steam. That's another option I suppose. I put a new cap on hers and it was good from there.
Last edited by Zander916; 06-30-2013 at 09:48 PM..