old p6sba question - 12 beeps?

Hi. I've been running a p6sba board with a P2 233 stuck in it for the best part of ten years now, with no problems. Recently I've purchased a P2 400 which I'd like to put in instead, but I'm encountering problems. Specifically: when I turn my system on with the new chip installed, I get 12 error beeps. The manual only tells me what the error codes mean up to 10 beeps. Help!