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).