OK, here's a shot in the dark. I worked on DesertHB's Xterra, and found the timing belt really loose. Could actully take it off without loosening anything! He was also overheating. We thought we were going to have to take the heads off, etc.

Anyway, we ended putting a new water pump on it, as the old one was questionable, some play and whatnot in the shaft. Then we installed a new belt, with the proper tension on it, and zoom(!) it not only ran better, but didn't get as hot any more. When we ran it up hills, a/c on, dogged it, et cetera, she got warmer, but never like she did before.

I'd wash out the a/c condensor, but double check the tension on the t-belt. A new one is cheap, and if does fix it, then we're on to something.