I was doing some research (before) to see if I could get my h264s playing back in MC with DXVA (old laptop here), and found that the MS Video decoder likes using DXVA for video acceleration. Not that DXVA is bad in itself, but ATI's video cards, while officially supporting the h264 main profile, does not support anything more than that. As a result, even slightly over-specced input can cause the display card to choke (which is what I observed) and giving the blocking/gibberish for those few frames. After the data stream goes back to standard, and the card's memory is replaced completely with fresh information, the situation abates.
I'm not sure if this is completely to blame in this case, but I thought I'd share what I know. I'm using software decoding almost exclusively now.