Why does my cheese sauce solidify I am using this recipie to make cheese sauce for my maize crackers.
http://allrecipes.com/Recipe/nacho-cheese-sauce/detail.aspx

However, my sauce starts solidifying as it cools down. I am using a slice of provola instead of american cheese. could that be a problem? 

Next time I tried doubling the milk andadding a little more flour, it fared a little better, but suffered the same fate. I need it to stay liquid for at least 90 minutes after removing from heat
Yes substituting provolone makes the difference.  Since American is a processed cheese, when it melts it doesn't become stringy but just goop.  I imagine that the recipe is specifc to American, maybe something like cheddar would fair well too.  But something like provolone which is close to mozzarella will become cohesive with itself as it cools.