So lately I have had problems with my chain coming off the middle chain ring when I climbing a hill. It only seems to happen when I'm mountain biking and not when riding on the street. Some people have told me that the chain ring is mostly likely worn, but it has under 300 miles on it. So I guess my question is what other causes could there be. I'm riding with a Sram cassette, KMC chain, and a Truvativ Firex Crankset. Any help would be great.