The reason to run the HDMI video through the 990 is switching - many HDTV's only have a single DVI or HDMI input (although two inputs are becoming more common), making it impossible to connect two DVI/HDMI sources. In your case, you ahve both the Motorola cable box and the 981HD. The 981HD has to use HDMI, since it lacks a component output.

There has been a fair bit of discussion lately about the various problems with Motorola cable boxes (Scientific Atlanta's not much better, although as of late Moto seems to be having all the major problems). It's possible that the Moto's HDCP implementation can't deal with switches between it and the display, thus making it impossible to get an HDCP handshake. Maybe some other Motorola users can chime in with some feedback. In the meanwhile, you might experiment with the order in which you turn things on - have the 990 and TV on and the 990 on the correct input before you turn on the cable box, for example. It also never hurts to make sure you have everything hooked up right and set up right in the menu (in case you connected it to DVI2 and picked DVI1 in the menu, for example - I've done that with component inputs on the 990 before).

I know the 981HD has no problem with this because I'm using mine with the 990's switching, but I switched to component output for my Pace Micro HD cable box because it occasionally had HDCP handshake errors that bugged my wife - and in that case, at least, component output looked just as goodas DVI.