Pretty rare for this to be a problem that's traced to the size of the vent, usually when there's a venting problem on a Bandit, it because something isn't right with the system, as in being clogged, pinched (I know - you said you checked that), or the vent line has a problem inside the tank (usually causes fuel to leak out the vent hose until the tank reaches a certain level).
There's been a B4 or two that have had venting problems when a tank bag is covering the cap, but I've yet to hear of any reports of this on a B6 or 12... but even this usually indicates there's something wrong with the vent system.
One thing you could try when it starts acting this way - open the cap and listen for a whoosh or hiss - that would be a sign the tank isn't venting well. If you ride around a bit with the cap unlatched (as a troubleshooting measure) and the symptoms disappear, you know it's a venting problem.