Strange sink behavior -- sink fills partially then drains at a normal pace

Question

Our bathroom has two sinks of the type where the entire basin sits above the countertop level. For several months (at least) now, the sink I normally use has filled up to about 2/3rds or so of its level when I first use it in the morning -- once it reaches that fill level though, it starts to drain normally and continues to drain normally for the rest of the day. I looked under the sink, and it is P-trapped; also, other drains behave relatively normally in comparison, save for the adjacent sink, which also drains slowly at the same time as the sink I normally use.

Why is this going on? I'm stumped -- the only theory I can think of is a venting issue, but that wouldn't explain why it only occurs for these two sinks, but not the bathroom tub (which simply is sluggish to drain), or the washer immediately on the other side of the wall (if water couldn't get through the trap there, I'd expect it to go up the standpipe and start spilling out the top, and it hasn't).

Answer 1

My first thought was venting as well. The washer and tub may have different vents and have no bearing. I'm assuming, of course, that you tried the simple approaches, like pulling out the drain stop to ensure it isn't clogged with hair and gunk, and dropping the trap to ensure it is clean? I had an issue like this with a tub drain where the vent was clogged, and there was no way to get to it. I worked at it with a hand snake and plunger for a while and it eventuially opened the clog.