Temperature Gauge Help?

I've just purchased a Ford Focus 2001 1.6 LX with 56,000 miles on the clock. Driving around town everythings perfect, couldn't fault a thing, however when I went on the motorway recently, the temperature gauge started playing up. At a constant speed of 70-75mph it stayed, as it always does, in the middle between cold and hot. But as soon as I hit 80mph it flew into the red? I slowed back to 75mph and it went back to the middle! I kept going from 75 to 80 and back to 75 and it just kept doing the same thing? Anyone have any idea whats going on at all? I'm pretty sure the engine wasn't overheating as after 1hr and half of driving i felt the bonnet and i wouldn't even say it was very warm?
Your advice would be of great help!

Lewis
August 2010