Recently two of our toilets have started to backup, we plung them and then they are back to normal. One toilet is a recent install, including all the plumbing and has been worry free for over two years, the other is on origianl plumbing and has been fine since we moved in. Only in the past several months have we experienced this, and not every day, more like once every other week or so. The last time the toilet on the original pipes clogged, it drained real slow once it flushed, and would act as though there was a vacumn in the system as the water in the bowl would slowly drop below the opening in the bowl.

We are on a Septic system that has two branches exiting the house. The venting for the new install is good, since the tub. sink. and shower all drain quickly. Could this be an issue with the tank?