Hi
Can you post your config here ?
From my personal exp we had some customer sites badly affected with nachi,blaster and their variants which was mitigated using route-map.
in which we mention or configure the certain match criterias and drop the packet inplace of forwarding it.
you can also check out the port details with which you are getting the traffic sourced or destined,that will also help u to isolate whether its some worm related traffic or something being done purposefully.. (thats getting ur traffic blackholed)..
if u want to lookout for a upgrade do post out ur current ios version o/p..
regds