My main floor bathroom sink drains slowly with typical intermittent use. If I let it fill a couple inches deep with the drain open, however, it will sometimes "let go" and quickly drain, even with the water on full blast. As long as I leave the water on, the drain keeps up fine while making a sucking sound. If I turn the water off and return later it's slow again.

I checked the trap and ran a cheapo crank snake out about 12' which came up clean. I also got on the roof and ran a hose down the vent pipe for a while. It did not back up.

Thanks for your help.

When you drain are there any gurgles or bubbles? Back to you, Tom