making brittle at high altitude

You'll certainly need to cook it longer because of you elevation. For every 500 feet in elevation above sea level, water boils at approximately 1 degree lower in temperature. At your elevation, water boils at between 201 and 202 degrees F. I imagine you've noticed that pasta takes longer to cook than package directions typically indicate, right? And in the early stages of cooking any sugar and water combination, water is what is being driven off once the sugar crystals have dissolved, and can then begin to store energy and cook to the deep colors that you see in beautiful brittles. Seizing, I suspect, however probably has nothing to do with you elevation. Rather, it sounds as though your solution crystalized. There are some good ways to prevent this. First, be sure your pot is absolutely clean before you begin. I use a copper pot for the sugars I cook; I scour it with salt and a halved lemon, then rinse it thoroughly. If you're using a stainless steel pot (a good, heavy-bottomed one is best), similarly scour and rinse it well. Second, you need to be sure that there are no sugar crystals clinging to the sides of the pot once you have stirred your ingredients together. I like to run a rubber spatula firmly around the sides of the pot a couple of times. Beyond that, as the contents begin heating, you need to either dip a pastry brush in water and hold it just above the line of crystals you can see, letting water run down the sides of the pan, and as it heats, melt and wash the crystals down into the pan. You only need do this once or twice at the most, otherwise you'll be adding unnecessary amounts of water to the pot. Alternatively, you can set a lid on the pot, which will cause condensation inside once it comes to a boil; the condensation will do the same job as the pastry brush and water. Once the sides of the pot are clear of any sugar crystals, be sure to remove the lid from the pot so that water can evaporate; otherwise, it will take approximately forever for your sugar to ever begin to caramelize. One last thought: it is imperative that you not stir the solution once it comes to a boil and as long as it on on the stove. Stirring can promote crystallization, so wait until after you've removed the pot from the heat to stir anything into it.

Here's what I could find on troubleshooting candy like brittle,
http://baking911.com/learn/baked-goods/candy/types/toffee

Since you are at high altitude, you will have to adjust the cooking temperature,
http://candy.about.com/od/candybasics/qt/Adjusting-Candy-Temperatures-For-High-Altitude-Cooking.htm

Here's what I could find on troubleshooting candy like brittle,
http://baking911.com/learn/baked-goods/candy/types/toffee

Since you are at high altitude, you will have to adjust the cooking temperature,
http://candy.about.com/od/candybasics/qt/Adjusting-Candy-Temperatures-For-High-Altitude-Cooking.htm