I agree with Gonk, check the 990 setup to make sure everything matches how you have it hooked up to the back of the 990. One of the things you could try is remove the comcast box and check out the operation of the 990, DVD, and your Panasonic display. I have also had to replace my cable box (Scientific Atlanta) due to quality issues so lets find out if the cable is the problem.

I'm not sure your issue with inconsistent sound, either from the TV or the surround processor, is the same as I have experienced since you have you are using DVI/HDMI cables. I don't use digital video as my plasma only has component inputs. I didn't think DVI includes audio. Regardless, revisiting the 990 setup might resolve the matter.

Keep us up to date on your progress.