hi ...
Check whether the ammount of missed packets are also increasing 'show interfaces FastEthernet?/?'. If this is the case then your IPS might be increasingly overloaded. Please be aware that a 4250 can analyse up to 350 Mbps .. then it will start dropping packets and performance will decrease. Also you can check your signatures and retire the ones that you consider irrelevant for your scenario. This will remove the signature from the signature Engines and will inprove sensor performance. This can also be done for sigantures that might be firing up too many false positives alerts.
I hope it helps .. please rate it if it does !!!