you can try to see if it is a ram problem by rebooting the server.. if after reboot the first mobs spawned act normally and then after a few hours you have this problem then it's most likely going to be a ram problem because mobs get spawned a few at a time for different hours.. also you could ctrl + alt + canc and open the task manager to see how much ram you have and how much ram the computer is using.. if you are near the limit it would be better to upgrade to a higher value or lower the number of mobs spawned in the mogen..