They may have squeezed HBO in with some other channels.Cablevision. The other HD signals are great. Just HBO is awful.
Cable providers like Cablevision have 38.8Mbps available per QAM slot. As distributed by satellite, HBO, Showtime, and Starz all require 10-11Mbps average and about 14.5Mbps peak (i.e. during action).
The bean counters at cable companies think it is wasteful to stick HBO and SHO on one 38.8Mbps QAM channel, because that only uses 20-22Mbps average and 29Mbps worse-case. The remainder of the 38.8Mbps is unused.
A number of cable providers, including Cablevision, Cox, and Time Warner have installed rate shaping equipment to squeeze a third channel -- say, HBO, SHO, and Starz -- into the same 38.8Mbps QAM slot. When one of those channels need more bits -- such as during action or movement -- pixelization and blocking results because the needed bandwidth isn't available. In this case, the provider has prioritized quantity over quality.
I suspect Cablevision added new HD channels at around the same time this problem began.