The battery reboot works. I suspect that the issue lies with the on/off switch. If you very deliberately turn the switch on and off, it works better. A sloppy attempt at turning it on or off can result in the keyboard (MD-1121) malfunctioning. This is too bad as the sounds on the keyboard are excellent. Rebooting requires removing the batteries for about 15 minutes before starting the machine up again (with batteries to keep memory and wall power). I'm going to try replacing the switch. I have three MD-1121s, one works perfectly, one has the rebooting issue, and the other stopped altogether. I'll report back.
Gary C, March 2018