I went through a similar thing some time ago with my tank. Eventually, I upgraded my skimmer and the problem went away almost immediately. I had don't all the same things that you metion trying....That's my experience, FWIW...
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400 gal reef. Established April, 2007. 3 Sequence Dart, RM12-4 skimmer, 2 x OM4Ways, Yellow Tang, Maroon Clown (pair), Blonde Naso Tang, Vlamingi Tang, Foxface Rabbit, Unicorn Tang, 2 Pakistani Butterflies and a few coral gobies
My Tank: http://www.canreef.com/vbulletin/showthread.php?t=28436
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