Heating up / Overheating under load

[Josh]

So I was on a trip to Seattle yesterday and I was going about 85ish up I-5 on a hill and I looked down and my needle was a little more than 3/4 the way up the temp gauge. I let off the gas and coasted for a while and it started cooling down. Now whenever I'm putting load on the motor above like 60mph the temp gauge starts to climb. What can cause this? Broken clutch fan? Bad coolant? Blocked Radiator? Anyone ever experienced something like this? I have quite a bit more driving to do but it doesn't seem to be doing it if I take it easy on the gas pedal. I'm not really sure what happened... it was working fine before this happened, I'm not exactly nice to the go pedal all the time but its never overheated...

Thanks!