Since the same question keeps coming up again and again in this fourm:
Read the
The "Send As" right is removed from a user object after you configure the "Send As" right in the Active Directory Users and Computers snap-in in Exchange Server
Should give you a better understanding of what the problem is and then you can device your own stratergy to fix it.
KB 912918,would do the trick ,but I recommend you read 907434 to understand the probelm better "where permissions are revoked in 1 hr"