WHen running at speed up a mountain, the fan isnt going to help you a lot (it's for low speed driving and idling).
Some of the parts store stats out there are not balanced. So when you're on it (like going up a mountain road, at say 3-5K RPM), the stat doesnt necessarily open when it should (it can be quite delayed).
Because you have to ask, I suggest changing the t-stat. A Mr Gasket is what I like (10 bucks at most speed shops, or under 9 bucks last time I checked Summit). It is balanced, has the jiggle valve, flows better and opens fully sooner than some of the stuff at the parts store (when you hold them up side by side, you'll see).
Also make sure you have a spring in your lower radiator hose, on the small chance that you're collapsing that hose (assuming your driving up the mountain is spirited).
Now if you want to get an idea if the stat is opening, you can feel the upper radiator hose (be careful, as the outside of the hose gets hot). It should get pretty firm at the stat's rated temp. You can remove it and put it on a pot of water on the stove, but if you do that, why not replace it anyhow.
See if you have the little air dam under the car.
Good luck.