Same here, Z400, 6GB RAM, NVidiaQuadro FX3800, MojoDX, Win7 64bit SP1, 275.89 drivers, QT7.7, MC6 Plays back flawlessly without the Mojo attached. As soon as I connect it, playback is limited to 1-2secs top, then the picture and system freezes. It will allow me to force quit without restarting. Looks like a serious bug to me, but this being Avid, I'm