I am not using audio - just VST instruments on MIDI tracks with FX plugins.. I do have a dedicated drive for audio and to save projects to though.
Audio device is M-Audio Ozonic.
"stuttering etc can sometimes be caused by another device locking out the sound card. " - well I don't have this same problem when using Cubase!
"have you run dpc latency checker ?? " - No sorry I will look into it - never heard of it