Yellow flame definatly sounds like a lack of air. Like already been said, check the burners for bugs, and raise the pots. I had the same issue with mine. I raised my kettle about 2 inches, and it works like a charm.
Thing with a yellow flam, you are not getting all the potential heat, and wasting fuel. THe hottest part of the flame is the white / blue. So, too much yellow, = wasted fuel.