Something doesn't sound right here!
Temp should not fluxuate that much during normal driving, even going up hill you should not flux more than a few degrees. I could understand if you were hauling a 5,000 lb. load uphill.
Have your cooling system flushed lately?? Sounds as though there might be a blockage somewhere or your fluid is low.
I just finished hauling a car over to the body shop with my Rodeo (3.2 motor) going up and down all kinds of hills(nothing is flat in Vermont)and the temp did not flux more than 10 degrees all the way.
John