When I have this sort of problem, I open up each component individually, inspect the circuit board or hand wiring with a magnifying glass, looking for defective solder connections, broken circuit board traces, and dirt. I repair anything I find to be defective, and clean up the dirt. I swab the input and output jacks with a q-tip and solvent and usually find a surprising amount of dirt and corrosion there. Next I check connecting cables and often find they need replacement. These things take a lot of abuse and poor quality cables cease to function well rather quickly.
If the problem persists, then I set up my workbench to do a signal trace. I put a signal generator into the input of the component and using the probe on my oscilloscope, I can monitor circuit function through each step. When I find the problem circuit, then I can test the individual components,(capacitors, resistors, solid state devices, tubes, etc.) at this point the problem has always revealed itself and I can make the repair.
I should mention that I rarely have to do a signal trace because the problem is usually fixed when I perform the tasks in the first paragraph.