High CPU Usage on Server (2008R2)

Hello,

  For the past three months, one of our servers has started to spike in CPU usage about half an hour after new virus definitions are downloaded (on 12.1 RU5).  It typically lasts around ~15 mins but it's lasted as long as an hour.

  I don't run an active scan when new definitions arrive and I haven't changed any settings on the server or in SEPM, and it only seems to be affecting this one server.  

  Anyone experience anything similar or have any suggestions?

  Thanks.