1998 Ford Escort overheating only going up steep incline

Engine Cooling problem
1998 Ford Escort 4 cyl Front Wheel Drive Automatic 131K miles

owned since new. Properly maintained by dealer only. Small coolant seep diagnosed as waterpump. Replaced it and hoses and while we were at it did preventative. Changed belt, tensioner, thermostat, radiator fan/shroud, fan switch, fuse, relay. Still leaking waterpump. Dealer replaced again. Got it home. Still leaking waterpump. Dealer fixed again( 3rd time ) this time no leak. But then started overheating. Never overheated before even when it was leaking. Strangely enough it only overheats when driven UP A STEEP ANGLE. We live in the ozarks. We can drive it around all day and the needle doesn't even move until we go up a STEEP incline, even a SHORT ( 1/4 mile) STEEP INCLINE makes the needle go to the end of the white. We stop it before it gets to the red. No signs of fluid loss at all. No smell. Doesn't seem to be unusually hot to me. Everything else on this vehicle works like new. This was the first problem we have had with it since new.