Cut and paste of an earlier post of mine:
If you get spikes with just a few tracks playing it might be a bad driver - try running DPC checker: http://www.thesycon.de/deu/latency_check.shtml while Cubase is spiking. If DPC checker is complaining (by displaying red bars) try to find the culprit with http://www.resplendence.com/latencymon (also while running cubase).