RouterOS 3.x Gateway Stop Responding

I have the following situation:
My ip pool is out of ips, so i decided to add another pool 81.25.255.129-81.25.255.254
I have added a second ip (81.25.255.128/25) address to the interface where dhcp server is
added a new network in dhcp-server 81.25.255.128/25 and created a new pool 81.25.255.129-81.25.255.254
Dhcp server gives out ip addresses from the new pool, and the traffic goes too.. but after 20-60 mins the gateway stops responding, and i dont know what is wrong…
plz help

81.25.255.128/25 is not a valid IP to use for the router. Try 81.25.255.129/25.