Hi! And thank you for your donation it is much appreciated. The reason the server crashes is theorized to be because there are too many active monsters at one time. From examining logs due to crashes it looks to be due TOO MANY ACTIVE MONSTER which can cause the monsters code to go into infinite loops trying to determine it's next behavior. There is no easy solution unfortunately as this would likely involve rewriting every monster in the game.
There is a better solution that should improve stability in the future. It involves only spawning monsters when it makes sense instead of ALL MONSTERS. This will give overall better performance and lead to very interesting gameplay. This new mod I'm working on is VERY BIG and when it releases it will have the biggest effect on the server's stability, performance and gameplay. Monster Siege will no longer be a memory game