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?