Found this help tip on the web.
"My airgap on my sink keeps spitting water every time that I run our dishwasher. What is wrong?"
Quality made air gaps are pretty basic products that rarely go "bad". If your air gap didn't spit in the past then try running your garbage disposer for a few minutes. Basically the way your dishwasher airgap works is that water is pumped through the smaller side of the airgap (5/8") into the airgap and then gravity takes it to your sink (or disposer) with larger hose (7/8"). The larger size has an area of about twice as large and with gravity there normally isn't any problem. If the 7/8" hose is sagging or there is grease build up or a partial stoppage or blockage in your sink/disposer the airgap can "spit." After inspecting under the sink to make sure the 7/8" isn't kinked or going up hill at all (it has to go smoothly downhill all the way) then disconnect the 7/8" hose at the sink/disposer and point it into a large bucket. Then run the dishwasher and see if the airgap still spits. If it does then you know you need a new airgap and if it doesn't then you know the "problem" is after the hose.