Peter, I would be curious if the power plug to the tender sound board is making/not making contact because the wires have a lot of tension from the bundled wrap inside the tender and pull on the contacts. You might want to check the plug the next time the tender acts up. I had to pull a little extra gray power wire from the bundle and re-plug it into the sound board. It is fine now.
I would not go over 18 volts on Legacy equipment. I run my ZWL at 17 volts with the 990 Remote.
I like my eggs fried, but not my engine boards.
Sincerely, John Rowlen