change the ip inspect tcp reassembly queue length and test.

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124tcr/tsec_r/sec_i1ht.htm

To set parameters that define how Cisco IOS Firewall application inspection and Cisco IOS Intrusion Prevention System (IPS) will handle out-of-order TCP packets, use the ip inspect tcp reassembly command in global configuration mode. To disable at least one defined parameter, use the no form of this command. ip inspect tcp reassembly {[queue length packet-number] [timeout seconds] [memory limitsize-in-kb] [alarm {on | off}]}

no ip inspect tcp reassembly {[queue length] [timeout] [memory limit]}