ok, in thinking about this a little more I have the following thoughts.
1) This oocurs usually at the same time every night but at different times for everyone. Mine goes out about 8:10 which coincidentally or not is the same time I initially installed the cablecards with comcast.
This implies to me that each day the cable card tuner goes out onto the internet to validate itself and the machine it is on and it does that based on the time it was installed.
2) Stopping and restarting the receiver service removes the content restricted message.
This implies to me that the Windows Media Center Receiver Service has lost the ability to communicate with the tuners. This would also explain why I have problems taking control of the tuners when I try to watch tv on a third extender. Once the tuner has gone out and reverified itself the Media center receiver is unaware of this sees a failure and blocks the content. stopping and restarting it enables it to communicate with the tuners and know that now everything is ok and it allows the content again.
So in summary something has caused the receiver service to fail in its communications with the tuners.