If your solid state Williams displays slowly fade on and off in a scrolling
fashion the problem is very likely the two capacitors, C1 and C3, on the
power supply board. These are 100uF 150V axial electrolytic capacitors.
Replace them both and your problem should go away.
If your electronic supplier doesn't have an exact replacement for these
capacitors, you can go to a higher voltage such as 160V with no problem.
The higher voltage rating means that the capacitor will take a higher
voltage before it breaks down.