Duane
one possible reason is that packets are arriving out of order, which - in your IOS version - get dropped by ip inspect (aka IOS firewall). Support for proper handling of out-of-order packets was added in IOS 12.4(9)T2 (cfr CSCse42817) so if possible I would suggest trying a more recent IOS version - I see you have 12.4(20)T on the flash as well, or at least the config is referring to it, so maybe you can give that a try to confirm whether that resolves (or at least improves) the issue.
hth
Herbert
