I have one drain in my bathroom that drains normally, but every so often it won't drain until a certain depth is in the basin (sink) then drains keeping that level in the basin. After it drains, or when the water flow slackens a bubble will come out of the drain and then the water will all drain from the basin. It's not repeatable, but I wonder if it might also be responsible for my occasionally slow to drain toilet.
Is this what a venting problem does, or am I describing something different?