Intermittent signal is usually a case of a loose connection or a short. I see that R5 is socketed. If it works loose, that could cause an open circuit. Check that. Next, try re-soldering all joints. ( Remove power and the IC and transistor first.) You may have a cold solder joint that is turning on and off.
If these fail, try using an audio probe to trace the signal path to find an open or short.