Have you tried this card on other computer??
Or, is the same dish used for a regular satellite tv receptor?
Just to confirm it is a Mythtv/LinuxMCE issue and not a bad signal from the dish.
Also, have you checked this wiki: http://www.mythtv.org/wiki/Hauppauge_WinTV_Nova-S_Plus_PCI
It says something about a bug on this card related to hardware problems with 22KHz tone generation for DISECQ signalling.
Maybe it is worth trying to apply the patch.