Hello and welcome.
Perhaps dirty switch contacts or, more likely the printed ribbon cable has a fault, which connects to the switch terminals. The Mk5 is not difficult to work on, so if the particular circuit in the ribbon is damaged, you can make point to point wiring and solder to each connection.
Here's a link to the manual which contacts schematics.
https://www.vintage-radio.net/forum/...2&d=1421628490