I first tired to use DWC as my preferred grow method, and quickly changed over to drip. My air stones would clog constantly from the organic nutes i was using, my drippers started to do the same thing but hey were easier to clean, and if i stayed on top of it they never clogged. Like u said, take the air stones off and see how much is air is making it through the lines, your problem may be the air stones and not the pumps. Since your running a long air line u may want to get a strong pump either way, and use bigger air stones.