OK, I did this Sunday on the highway to the trails (80 mph-see sideways photo): and the radiator overflow filled and then would not return the antifreeze to the radiator after temp dropped a little. The hose from the radiator to the overflow bottle appears fine. The only time this happens (3 times) is when I'm on the highway doing 70-75 or so. In the photo my speedo is off by about 5 mph. This is an F motor with a stock fan and radiator. The temp outside was about 85 degrees, so not hot and the temp gauge showed normal on the highway, and showed it getting hotter after the the fluid did not return to the radiator. I have had no other cooling problems in the past 2 years. Any ideas??????