I had a similar problem when setting up my front projection system and thought the DVI out on the 990 was not working.

I have a blu-ray player and a satellite box hooked up through my 990. I was not able to get any video to my projector when I ran it through the 990. I tried replacing the HDMI/DVI adapter, as this is a common point of failure, but that didn't solve the problem either. The odd thing was, every once in a while, when fiddling with the cables, I would get a brief (but highly degraded) picture from the projector. Like you, hooking my sources directly to the projector was not a problem at all.

I contacted Outlaw and they suggested I use shorter HDMI to DVI cables (1 meter or less). Being new to the HT arena, I thought this was an odd suggestion, but I purchased two 1 meter HDMI to DVI cables from Outlaw and that solved the problem.

So, you might want to try using a shorter HDMI to DVI cable from your cable box to the 990.