It wouldn't surprise me you're having problems due to the insufficient transfer rate from the DVD drive.
To help prevent piracy (sic) it's common for DVD firmware to be slugged to a maximum of 2x read speed, which is something like 2.5Mb/sec IIRC which I think is far lower than is needed for hidef which isn't compressed with something like WM9, AAC or similar codecs.
You can often find firmware hacks which remove the speed cap, as well as multi-regioning and other annoyance removal.