What kind of valve do you have on the air line going to the venturi? Some of the cheaper plastic valves don't have a fine enough adjustment to regulate the air correctly. I've never used a Pro Clear skimmer before, but my Red Sea Berlins that I used to use needed better valves than they came with to work right. Try a good quality needle valve. Even one of the better quality ball valves might work although I've never tried one on a skimmer.
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