Are you turning off your cable box/DVR in between your viewing sessions?
I have a DirecTV DVR connected via component cables, and had a similar problem for a while. I think the slingbox sometimes messes up detecting the incoming video format, gets confused, and then displays nothing.
With the DirecTV DVR when I power on the DVR I often have to hit the "Format" button (on the virtual remote) which refreshes/changes the DVR's output resolution. Then suddenly the screen appears.
Your ATT DVR may be having the same issue, but I don't know if you have the ability to change your display setting with the ATT remote.
Hope this helps!