While water is running full blast, bathroom sink repeatedly fills to about half and empties. Sometimes it drains normally

Drains are covered in a second uninstalled bathroom with two vanities.

We're trying to get our Certificate of Occupancy for this new house and worried that this could cause it to fail.

Describe the bathroom sink. Is it a regular lavatory or a vessel sink? Any other fixtures draining such as a washer when this happens? Cheers, Tom

Tom--Regular lavatory with no overflow hole. It happens when no other water is draining. Could it be something to do with the p-trap, etc. With that off, we flushed water directly into the drain and it did not back up. --Sharon

Please note that this is new plumbing, used mainly for testing, just turned it on 2-3 days ago.

While water is running full blast, bathroom sink repeatedly fills to about half and empties. Sometimes it drains normally
I'm confused! Are you saying that you have a pop-up waste that's closed when you start to fill the lavatory and it opens up and drains when the faucet's running or are you attempting to to fill the lavatory with the drain open? You're going to hafta give me aomething to work with here. Details! I need details. Regards. Tom