Problem Lag Spikes/Server can't keep up



I've been getting lag spikes every 2-3 seconds since I started playing the pack, and the server console keeps giving the "can't keep up" error. I'm running the server with 6gigs allocated and playing the game on the same machine (I've done this many times with heavier packs so I don't think my computer not being powerful enough is the problem).
The lag spikes get worse and worse the longer I play, and the only way to momentarily stop them is to restart the server.
Does anyone know what is causing this?
Could it be the Java version I have installed?