Is there a ring around the neck? If so, definite infection.
I used to have a gusher problem, then I followed the Papazian recommendation of soaking all my bottles (I use Grolsch-type) in a bleach solution (1 cup in 5 gallons) overnight, scrubbing them with a bottle brush and a hot water rinse, sanitizing and then filling! It worked! I am currently enjoying my batch of British Bitter and it's infection free!