Great game! very balanced once you work out the lag issues :) from what my gaming community and I have experienced so far, it's probably just script error... because the spikes are there regardless of graphics settings (except shadows, turning those off helps a lot) and hardware. And none of the lag spikes are synchronized with other players so it's definitely not server-side. Keep up the good work though!
P.S.: Absolutely LOVE the easter egg at the main menu ;)