Toshiba SD-V393 VHS/DVD Combo shutting off?

The unit works great as a tuner, and as a VCR, but whenever I try to play any kind of disc in the DVD player, the unit suddenly shuts off. You can't even restart the machine with the power button. You have to unplug the unit for several minutes, then plug it back in and quickly open the DVD tray to get the disc out before the machine tries to read it and shuts down again.

Does anyone have any ideas? I think this machine is well worth repairing but I don't even know where I should begin to look to fix the problem.

Thanks!
Jon