993 is a "missed packet" alarm. Containined in the alarm details you'll see a details of missed packets. This alarm will typically fire on an over-subscribed sensor ... too much traffic at times to inspect, thus some packets are dropped. It's very possible that you are exceeding the rated capabilities of the device.
One thing you should definately do is upgrade to the recent service pack 4.1(5)S189 and then apply the S190 signature update. The service pack provides some bug fixes as well as performance enhancements. The current signature package available is S190, you're running S91 - you're almost 100 signature update packages out of date.
You can get the needed files from the following link provided you have a valid service contract, and a valid CCO login ID:
http://www.cisco.com/kobayashi/sw-center/ciscosecure/ids/crypto/
