Do you have anything else competing for bandwidth with Netflix -- other PC's on your home network, an old 10 MB hub trying to manage congestion, firewall or router settings with excess packet filtering, voice over IP (VoIP) / digital phone that is prioritizing voice traffic over data, Tivo or ReplayTV pulling down updates?
I don't know about Metrocast, but I know Comcast doesn't guarantee either a full time connection or the speed of the connection. Sometimes it flies, other times it can be a dog. The joy of consumer broadband...