Constantly having to re-bind our Macs

Hi folks... I just joined this forum today in hopes that I can find an answer to an ongoing issue I am having with a majority of the Macs that I administer. All Macs are running 10.5.6 and are bound to AD. The problem is that often it will refuse the user's login attempt. In checking their AD account, it is not locked or expired. Un-binding and re-binding will correct this issue for a period of time only to do it again. We have been bound to AD ever since we upgraded from Jaguar to Tiger and never had issues like this. Only after upgrading to Leopard has this problem happened. My next step is to search through these forums for any clues that may help. In the meantime I was really hoping that someone out there has experienced this and knows what the fix is. Thanks!