Recently we remodeled our third floor bathroom which involved relocating the sink, toilet and tub. Since the remodeling, our new bathroom sink doesn't drain properly. At times it fills up and then slowly drains out, other times it works fine. (The toilet and tub drain fine and they are all connected to the same line that goes into the main stack.) I removed the sink trap and snaked the line about 6' which about where the sink line meets the line that runs into the stack without having any appreciable impact on the slow drain. Any ideas as to what the problem might be and how to fix it short of tearing out a ceiling?

Is this a regular lavatory or a vessel sink?