The interface up/down/up is usually caused by the client computer restarting the ethernet interface connected to the router. If that is the only device connected to that router interface, you will get that message.
The dhcp behavior may be client device related. I had problems like that with only a few specific devices.
http://forum.mikrotik.com/t/dhcp-server-renew-challenge/66256/1
http://forum.mikrotik.com/t/security-help-as-well-as-why-router-deassign-and-assign-ip/65654/1
The MikroTik crew had a go at my router, and think they have corrected my problem starting with V6.1. Maybe that is your problem too.