What do you have the motion blur “extra frames” setting to something reasonable like “3”. If you have it set too high, the “artifact” problem will be worse.
What video card do you have?
Windows or Mac?
HQ motion blur uses the OpenGL accumulation buffer which is not supported by all video cards. It may be that you need to update your video drivers (if Windows) or that your hardware just does not support it.
Alan.
wondertouch