If you're renting a server, you can address the issue with your server provider and request a transfer to a less occupied machine. I've done this on a couple of occasions. If this is running off of your own computer then you're either looking at a weak connection or hardware that can't uphold the strain of running two instances of the same game (Both client & server-side). If neither of these applies to you, then you're probably better off looking for another server or talking to the Administrator / Owner of that server about this. There's not much else I can really help you with beyond that, so best of luck to you in your endeavors of survival in the lag-induced zombie apocalypse.