I think it's stopped for now, still monitoring like crazy. It's a 5-gallon water jug, three piece standard bell airlock with a drilled #10 bung. I've noticed the vacuum effect with the temperature change, but in this case, I have a heater in the closet and everything is room temperature. I'm trying to restart a stuck fermentation, racked to secondary and added a bit more yeast and some nutrient. Right about the time the yeast ring appeared on the surface, airlock started acting up. Sorry I missed the replies, I was trying to keep the airlock liquid from getting sucked backwards! Kinda wondering if the yeast was drawing in the oxygen faster than crapping gas bubbles? I don't know, it's super weird!