I ran into the same problem. Everything would look great in the composition, but when i rendered the video out, the background would cut off. It didnt cut off if i rendered half res though. Lame.
Luckily, my comp had lots of crazy camera moves, and i animated the position of my motion tile background during one of those moves. That seems to have worked fine.
Any ideas why it would render fine at half res, and not at full?
Im on a quad core PC, windows vista 64bit, with 8 gigs of ram.
Full res is 1280×720.