I would have highly suspected that the vent in the p-trap was malfuntioning but if you have replaced it I am at a loss to help.
We had one for our half bath sink that caused us some frustration a couple of years ago. Repaced the vent and the problem was solved.
05 Fleetwood Discovery, 39J Model,
07 Ford Explorer, Sport Trac, V-8, 4 x 4 toad.
Served Proudly US Army 1959 - 2006, Retired Twice