Sink drain slowly drains until the sink is full and then it syphons out quickly. I took the P trap out and it drains fine straight down out of the sink . I hooked the P trap back to the tail piece and left it unhooked from the wall and the sink drains slowly. The P trap is perfectly clean. Help What do I do??

Sounds like your line may not be vented. You can buy an under the counter vent system that would help if that is the issue. Ask the people at your local home improvement store to check out the vent system.

Is this kitchen sink or a bath sink?