Given all things being equal, it's usually the cable box. However, it's good to be sure since 90% of the time everyone will point at someone else when they don't know and you usually have to prove it to them beyond a shadow of a doubt.
First, more details would be good. Exactly which SXRD model do you have, what exact cable box (and what cable company), and how do you have them connected? If you want to use the DVD player to test against, also knowing what that is and how you have it hooked up waould be useful. It's also important to know if you are sending the video signal through a receiver or other type of switch.