Pomodoro for teams

So our XP team has been using some twist of the Pomodoro technique for about a month now. I actually haven't found anything significant about using Pomodoros in a team, so I'm going to share what we're doing, and what problems we're having, to see if you have any ideas.

We use "one pomodoro to rule them all", which means one timer for the whole team. I've read this is an anti-pattern, but I don't see a compelling reason to use more than one pomodoro. We see a lot of advantages in having one pomodoro... everyone has a pause at the same time, so pair swapping is easier, team meetings are easier, etc. If someone's pomodoro is interrupted, someone decides not to participate in this pomodoro ("check-out"), they just wait until the next pomodoro starts to rejoin the synchronized timer. As a team, it seems like we're happy to use Pomodoros--our team communication is up, our productivity, our focus, our check-in/out is all better.

Our biggest problem with Pomodoro right now is in sticking to the 25 minute window--when the timer rings, inevitably someone stops, but most of the team keeps working for another 2 or 4 or 5 minutes. When the 5-minute pause is over, 3 or 4 people approach the story board and start goading others into joining them. This can easily make a 5 minute pause take 10 minutes, at which point we have a mini-standup meeting to verify that we're not blocked or overbudget on any story cards, that we're progressing well, and to verify no one needs extra help.

We've talked, as a team, about why people aren't taking breaks, why people are late to show up after the break, etc. Maybe this pain is why it's considered bad form to use "one pomodoro to rule them all". If we keep just one timer, though, what else could we do to fix our tardiness?