If wiggling the coupling switch (or vertical sensitivity knob, etc.) makes a difference, then the likely causes are either dirty switch contacts or a intermittent/cold solder joints. Neither of these would really require the service manual to address/fix. Careful dis-assembly of the case, and judicious use of a contact cleaner like Caig DeOxit might be all you need. If the switch contacts are open/visible, sliding a piece of isopropyl soaked paper between the contacts will do a nice job of removing oxidation and gently burnishing the contacts. Take a careful look under magnification at any solder joints associated with switches and assemblies that are subject to vibration/movement when used - i.e. where switches mount to PC boards, etc. Look for cracked joints, etc. It's generally not a bad idea to re-solder these connections.