[USR-TC] Vanishing NMC's

Hello, We have some monitoring software that checks to see if our gear is online via pings. It seems that the NMC cards on my USR TC chassies disapear to pings alot through out the day. So often that we had to remove them from our monitoring software (because it pages us if there is no ping reply).. Has anyone else had this problem? Do you know of a fix? This happens across 6 chassies. So I don't think it is failing hardware. -Steve