95 Mercury Villager Overheat in certain condition only?

It goes overheat when I drive up the (steep) hill while keep the speed at 35 -40 mph, even without A/C. In other road situation it just normal and well behaved. FYI, water pump (& timing belt) 20,000 miles old, while thermostat, radiator cap, and coolant are new. Odometer 190,000 miles. The electric fan works both high and low speed. What's the possible cause? What should I do? Please help. Thank you.