Minecraft Server Connection Problems?

Hi i have made a minecraft bukkit server that uses hamachi. Im having a constant problem of my friends occasionally get kicked. Theyve all added the port code to the end of the server address, but still they are kicked randomly. i have even had my friends laptop in my house using my internet connection and still he is kicked. My friends are not kicked instantly but after a while of us playing in the server they get disconnected. Then they have to wait however long till it lets them back in. Everything is up to date, and im wondering what i can do to fix this problem.