Sorry for the delay in replying, I tried a few more tests first.

The bad news is that even with a kernel build that didn't lock up on one
complete playback of the video, even when trying skipping during the video, on
a subsequent reboot and run I'd experience a lock-up.

The good news is that with current git head, even though I can experience
lock-ups, I can restart kdm successfully following a lock-up. Prior to 4.0.0
kernel I needed to do a power down restart to undo the lock-up.

Besides Linus git head kernel built with current Debian experimental gcc-5, the
other packages installed include:

libdrm - related packages at version 2.4.60-2
libvdpau1:amd64 0.9-1
mesa packages 10.4.2-2
xserver-xorg packages 1:7.5.0-1

At one stage I wanted to be sure that it wasn't a problem with the 3850HD video
card so I removed it and used the onboard Radeon 3200HD and experienced the
same problems.