Engine running hot

Mazda Protege, 2001, 126,000 miles, automatic

The engine runs hot when on the interstate (running around 70 mph or more), but is fine when city driving. Do you have any idea why this is or how I can identify the problem since it doesn't occur all the time?

Replaced the radiator last year - it was corroded. Haven't tried any repairs yet for the hot engine problem.