run ctrl+R to make sure this is lag and not low framerate... if it's the latter then reduce particle density first (since it's happening with spells) and see if that helps anything. If you get a constant good framerate... then you're getting lag spikes. If you run recount they have a realtime latency graph you can run just to be sure.. but... if it's not framerate there's nothing else it can be... check with your ISP and figure out what's going on. Switch if necessary.