This could be a venting problem. Many times when you do not have proper air supply or you have a clogged chimney, the flames will shoot out the front. What keeps it going up the chimney is the warm air rising principal. If the air cannot go up, because of clogged chimney or because of negative air pressure, it will come out the front. If the air is not adjusted right on the burner you could have a similar problem. This is something that is dangerous, get a specialist in right away.