Intermittent OS X Server / AD error

I have an XServe set up for file sharing with SMB, on a Windows network. I've bound the AD plugin to the domain, and set Samba up as a domain member. Users log on with Active Directory credentials, and setting permissions etc works perfectly. The problem is, every now and again it will simply stop authenticating users, and needs a restart. A user will attempt to open a share, and be told 'username does not exist' or 'access denied', even if they were opening the same share five minutes earlier. This suddenly starts happening after about 12 days' uptime, and only a reboot will solve the problem. Restarting Samba services sometimes fixes it for a few minutes, but then the problem reoccurs. It seems as if Samba stops checking against the AD, but there's nothing in any log files to indicate what is going on. Does anyone else have experience of this? (I've tried setting a Cron job to restart the server once a week, but it doesn't seem to work and even if it did, it wouldn't be ideal). Any input greatly appreciated.