temporary loss of 12volt power when turning off lights.

either a bad connection somewhere
OR
yes , bad converter

check the connections between the converter and the battery
clean them and tighten them
especially the Negative ground connection
if that doesn't fix the problem replace the converter

BTW FYI most converters are Not designed to be stand alone power supplies
they are supposed to be connected to the battery
especially for high current loads, like Jacks and slides