I've been having this problem lately too, but only the gas post on one of my kegs. Started happening a few months ago, the keg was fine until then. It was a frustrating mystery until last night when I noticed that the O-ring on that particular post was sticking out farther than the others. I don't know if it's the wrong size, but as noted it worked fine for a long time. That particular ring is blue, FWIW.
So, maybe replace the O-rings on your posts? That's my plan.