I found the issue on my site that was plagued with this.Maybe this is the problem for others too. See my post on the other thread…
http://forum.mikrotik.com/t/ros-6-38-serious-dhcp-server-problem/105259/1
UPDATE: Sorry this did not fix it. The problem returns about the 4day mark. But added some more info and think Ive identified the underlying cause which I dont have a fix for other than to reset the radio. Read thread above for more details.