Thanks.

Check this post: https://bitcointalk.org/index.php?topic=3889.msg669081#msg669081
Since another pool on the same DC reported similar hashrate drop I think that it was caused by some peering link suffered from attack or newyearing, but this lasted only a few hours and now it's normal again. Do you still experience this issue ?
Can you try reconnecting one of your miners and check if it's normal again or not?
Do you see any packet loss to pit.deepbit.net ?