If your receiver is patched, try turning the patch off, Press Menu, System Information, then enter 2004 on the remote. The CAS and BISS Menus will disappear. To enable the patch again just repeat the process.
If the problem isn't with the patch, you could try re-flashing your receiver, with the original firmware.
If possible try your card in a different receiver, if that doesn't work contact the firm who supplied the card and see if this is a common problem, or at least a known issue, you might have to buy a dedicated cam.
Does this occur at regular intervals, when they're doing an update, or randomly, in which case are you losing signal, or getting interference, try checking the signal level when this occurs, if necessary record a channel with your signal strength showing.