Radiator Overflow not working with PHOTO!

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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??????
Overflow does enter from the bottom and the overflow tank/bottle does not leak. The drain/real tank overflow exits just below the cap on the overflow tank. I'll change the rad cap because it is PO.
Any leaks near the top of your coolant system will prevent the "thermo-syphon effect".

So if you have a hairline crack in your top headertank or a loose hose-clip somewhere relatively high up (for instance) - air will get sucked in through this instead of replacement coolant getting sucked in from your reservoir.

Another possibility (which you won't want to consider yet) is a cracked head or leaking head gasket (allowing combustion gases to enter the coolant).

I suggest pressure-testing your coolant system and checking for leaks as the first step.

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