For soda, it has a much higher carb level then beer. Mine is at 30 psi usually, with 30' of 3/16" beverage tubing at 40 degrees.
Is your line kept cold as well as the keg? Is the second pour better, or are they all foamy? It sounds like the co2 is breaking out of the soda, so maybe a little leak in the "out" post of the keg is the cause. Did you try taking off the post (depressurize the keg first!) and inspecting the poppit?