1992 Mazda Protege Overheating

I have a Mazda Protege 1992 with 162,300 miles on it. It has been a great car and recently it's been overheating but not to the point where the needle is all the way to the "H." The funny thing is that when I drive it on the street and keep it under about 45 mph it never overheats? But when I drive it on the freeway about 5-10 minutes the needle starts going up pretty fast, but never to the point where it goes all the way up? One mechanic told me it was my radiator.I then changed my radiator and the problem persisted. One mechanic told me that it was my engine, but my car runs smooth. Is there anything else I can try. A fuse maybe? Help? Or should I drive to work on the street?