About 1 & 1/2 years ago, we had our bathroom renovated and the company doing the work replaced the wash basin, among other things. Since then, it seems our sink is sometimes slow to drain. While running water in it, it will drain OK, then suddenly stop draining until it's 1/2 to 2/3 full, then drain again normally. I don't believe it's a venting problem, since it was OK previous to their work. I have taken the trap apart, but never found "anything" in it. I did notice, that it appears, the drain pipe from the sink attach to the connection at the sheet rocked wall is a little smaller than the pipe in the wall. Could this create some kind of a vacum problem ? The sink has never come close to overflowing, but it's just annoying.