9 times out of 10, when I've had this problem with groups, it's come down to the drummer getting into what the guitarist is doing, he pumps it up a bit, and the guitarist reacts...... and the drummer pumps it up a bit more..... and so on. It's great to listen and react to each other, but not at the detriment of your primary function....... keeping time. Talk to the band about it, and see if a bit of focus on it during rehearsals doesn't cure it. One thing you can do is play the tune without the guitars, and see if there is an improvement. If there is, then again, a bit of focus from the drummer and guitarist can help. Also, having the drummer focus more on the bass line instead of the guitarist is a big help.