It sounds like some kind of authorization issue with the cablecards. When you have the problem, you can go into Messages and Setup->settings->cablecards and check the authorization and other values. This way you'll know it is the cards/headend authorization and can contact verizon about it.
Since it fixed "itself", my guess is it is a problem on verizon's end sending the auth to the cablecards.
Since it fixed "itself", my guess is it is a problem on verizon's end sending the auth to the cablecards.