For the last week or so only one of three toilets in the house "bubbles" in the bowl at random times. We are on a septic system pumped yearly. The toilet does not appear to be clogged because it flushes fine. It is located upstairs but the other upstairs toilet does not bubble. This problem started when it was -25F outside but I do not think this is the problem. What's your opinion? Thanks

Bubbles in your toilet bowl indicate that you have a partial blockage in the drain line. The bubbles are dewer gas that's being pushed back up the line when the discharge hits a clogt. You will have to snake out the drain line. Regards, Tom