I have a very similar problem with another brand (Eljer) that has slowly gotten worse. However, if you watch the opening of the flush valve when you flush large air bubbles enter the tank to impede the water from leaving the closet into the bowl. Any one have any guesses as to why air should enter from the drain side? This toilet is located on the first floor. Another identical model toilet is installed on the second floor right above the downstairs one and has no such air bubble or flushing problems. Does any one still think this is a vent problem?