Intermittent problems are often due to a loose connection. Once you find the connection, you can simply solder it. But how do you find it?
When the equipment is exposed and running – being careful to avoid shorting anything or shocking yourself, you can tap on components with the eraser end of a pencil. The badly connected component will act up when tapped.
Troubleshooting a printed circuit card is much easier if you isolate the circuts. You can harmlessly cut through a trace with a razor blade, and then solder a wire over it after testing.