92 F150 overheating problem

I just bought a 92 F150 that runs great except when I go up a steep hill it starts to get hot. when I am pulling my boat it overheats. i have replaced the thermostat and the waterpump is not leaking or making any noise. does anyone know what the problem could be it runs fine on level ground

[b]When climbing a hill or towing anything you’re demanding more power from the engine. And more power creates more heat. Because of the age of this vehicle, it could be that the radiator is partialy restricted.

To find out if this is the case, get the engine up to full operating temperature. Take a laser thermal gun and point it at the cores of the radiator. Those that have full coolant flow will show up hotter than those that don’t. From this it can be determined what percentage of the radiator is restricted. And thereby causing the engine to overheat under certain conditions.

Tester[/b]