Since you are in Denver you may be having problems with high altitude. This site will help you adjust your baking http://www.highaltitudebaking.com/adj_recipes.htm (high altitude baking).  If that isn't it you could be overcreaming the butter or using expired baking soda.

The only other thing I can think of: are you allowing the cookie sheets to completey cool before using them again?  A hot or too-warm cookie sheet can cause the batter to spread.

Well, I'll check my baking soda.  That could be expired.  I know it's not the baking sheets because I have four of those and if I need to make more than four sheets of cookies, the first one is always cooled by the time I finish baking the other three.  Hmm....

I started out living at sea level, and baked Tollhouse cookies with no prob.

Then I moved to high altitude and the cookies spread out and were thin.  I discovered I loved loved loved them that way.

Now I'm back at sea level and can't get the cookies to thin out the same way.

It's the high altitude factor.  You need to adjust your baking.

Also, sometimes chilling the dough first can help with this- not sure why though!

I find, also, if I use butter crisco my cookies come out softer.  You will have to adjust for high altitude though.

try adding 1/4 cup more flour.... those make mine fatter.

Generally too much shortening or too much sugar causes spreading, if it's not a hot-pan issue.