As it's intermittent, it makes life rather difficult - and a meter isn't going to help much.
What will help (and probably more than a scope to some extent) is a monitor amplifier and speaker.
This could be one channel of another stereo amplifier, or a simple little mono amplifier, such as an LM386 or similar.
Use the monitor amplifier to monitor the signal before it gets to the power amplifiers, and see if you can find a point where it begins to go faulty - the top of the volume control (assuming a mechanical volume control) is a common first point to check, and work backwards or forwards from there, depending if the signal disappears there or not.