I am having a problem with siphoning/misting from my fuel vents.  I have a 206F with a sportsman cuff, and wing-x extensions.  If I fly faster than 135mphias I see a heavy mist siphoning from my fuel vents.  Enough siphoning occurs that the underside of the wing is very stained and when opening the caps I have on occasion heard the sound of a vacuum breaking.  I also notice a fluctuation in fuel flow of about .5-.8 gph where the needle moves rhythmically about 2 times a second. This fluctuation is duplicated by my JPI fuel flow meter.  I have 4 new fuel caps which have open vents and flewible gaskets.  My vent positions were set as close  to book specs as poss. (with certain error built in by the poor fit of the aftermarket strut fairing which leaves a nearly .5in. gap to the strut) I have also  set the vent to several places in between.  I bought a new Cessna made fairing with only marginally better fit, and a questionable (improvement?) to the symptoms.  If anyone has any experience with this situation and a remedy I would love to here from them.