Infections can be strange. I think there's something going on with your beers besides just overcarbonation. I've gone to 5 volumes without gushers. CO2 comes out of solution much more quickly if it has nucleation sites. If the 5vol beer is well-settled it won't foam over, if the yeast is up in solution, it foams over quickly. So I guess it's possible your infection isn't actually changing the gravity or carbonation level, but the bugs can stay in suspension, creating nucleation sites and foaming over. Just a WAG.