While your tuner may have some issues, it is not unusual for the elements in a neon lamp to degrade, raising the voltage needed to fire the lamp.
I then functions as a relaxation oscillator, causing the flickering that you see.
Depending on the lamp, it may need something over 65VDC to fire. You can measure the voltage to the lamp on both sides of the series resistor.
One odd thing about neon lamps...If they are flickering and you shine a bright light on them, they may stop flickering and stay on.
The lamp is the first thing I would check by measuring the voltage going to it when the tuner is tuned to a strong stereo station. If that is low then you will need some tuner repair.