I believe that you loose signal even when using dvi/hdmi adapter because you use the on-board graphics. The chip does not care if you use the hdmi output or the dvi output, will react in the same manner when tv is switched to other source.
You may wish to try this: http://thydzik.com/tag/hdmion/
I am using the satellite receiver as I do not have a dvb-s card in the computer yet. I was planning to get a Hauppauge 2200, it has dvb-s and analogue (for Prime) but it appears one of them is not working under Media Portal, so i posponed the purchase of this card. I need the dvb-s as I need to get Optus B3. I already have the 65cm dish with 2x LNB for Optus B3 and NZ Freeview running for more than a year now. When I will find a card able to move all this into the HTPC, I move it. It would be so more convenient changing channels and scheduling recordings... and that will solve the EPG problem as well, as it will get it from the dvb-s signal, right?
back to your question: my HA in MediaPortal is null, as I get 40-45% cpu load. I just realised there is something wrong with my codecs and that will require some work... somebody (on another topic) pointed out to me that the Nova-T 500 HD might give you SD signal if the codecs are not right. And I believe this is what happens to me, as I get bad pixelation and artifacts when a smooth colour gradient needs to be displayed. All I get is concentric circles (bands?), each of a different colour, no shades inside each circle... you know what I mean...