OD users fail to login randomly, require server reboot

Hi folks,

I'm running OS X Yosemite server on an iMac and have 5 other iMacs as clients in a small office.

I have a weird problem with my OS X Yosemite setup. I have successfully bound all clients to the server and they can login without any issues. However, on random occasions, users fail to login to their workstations and the only way to resolve the problem is by restarting the server app or in some cases rebooting the server itself. It feels like the service stops for some reason. Please excuse my beginner knowlege level of OS X server. Any help would be greatly appreciated.

PS: I read somewhere on the net that unbinding the clients and rebinding them to the server solves the problem. I tried the suggestion but after a few days problem returns.