Engine Temp/Loss of Power

Can anyone tell me why when i go up a hill, my temp guage goes up, and i seem to loose horse power (specifically, the engine works hard to down-shift). But then, when i level out or just let up on the gas a bit, the temp goes back to normal and so does the power of the engine. When i first start up the vehicile on a cold day, the power is fine, even on hills, but when it warms up a bit, it starts to act up. Any tips help???

TKS!