Possible cam problem with mythtv and dvb card

My Satellite Setup
My Location
I have a mythtv box that I've set up to receive foxtel (australia) satellite using both a Twinhan Sat-CI and also a Nexus-S-CI card. I have an Irdeto 2.09 cam.
I have managed to get it working and decrypting satellite channels successfully. However after a day or so the decryption fails and stays failed until the cards are reset. After I reset the cards either by forcing a reset via the driver or by rebooting the machine the decryption works immediately.
It isn't reliable, sometimes it will fail within a few hours other times it may last 2 days before failing. It usually is noticeable upon tuning to a completely different channel, the tune will succeed but nothing will come back from the card and the streams are reported as encrypted.
Also tuning to TVSN will succeed and display fine after the failure has occurred.

Does anyone have any ideas on what may be causing the decryption failure? I'm leaning towards the cam itself but the cam still responds to information queries when its not decrypting successfully so it doesnt look like its completely dead/crashed.