I've got a 1990 Mazda MPV 4cyl. 5spd., used as a work truck that has a peculiar problem. The coolant temp. goes to 3/4, and sometimes almost at red zone, on the highway, doesn't matter if you're doing 50mph or 80mph. All other times, it is perfectly at half point, in blistering heat in stop and go traffic or idling for 1/2 hour. No loss of coolant, no overheating, drives really well. Gets almost 22mpg loaded with 600lbs of construction supplies and equipment. any ideas?