First... LED or Bulbs?

If LED, go to Adjustements / Standard Adjustements / Allow Dim Illumination - set this to off.

The 'bulb saver' technology that WPC machines use is really remarkable, but they dim the incandescent bulbs by shutting off power to them. LED's have a really obvious flicker effect... unless you buy the higher cost 'flicker free' LED bulb replacement from Cointaker.

The bulb saver technology uses some circuitry called 'zero cutoff' and I've always suspected that problems with the 'zero cutoff' section could cause inappropriate flickering, but I personally haven't seen this problem.

All your (controlled) lamps use the +18V, so a problem with a fuse acting like a resistor at F114, browned pins/plugs at J101 could starve the 18V of it's power. BR1 is the bottom Bridge Rectifier under the big silver heat sink, and a failure here could mess with your 18V. An absolutely failed C6 and C7 could cause you to not have smooth DC voltage, but I've replaced a crazy amount of failed bridge rectifiers and VERY few 15,000 uf capacitors.

Troubleshooting.

You did the right thing. The General Illumination and the 'controlled' illumination are different. Isolating the problem to only the controlled illumination side helps.

LED's and Dim Illumination setting are what happens when you are not playing... like fifteen minutes after you stop playing (adjustable) the lights would dim down by flickering.

In test mode / All Lamps, do the lamps come on brightly without flickering?

I would re-seat J101 on the right side of the big Power Driver board. I'd visually inspect the capacitors for leakage, and F114 for any indication that the fuse has been getting hot.

I'd re-seat all the grey ribbon cables (with power off).

Then (with power back on) I'd use a meter to find out if my +18V is reading correctly.

After that, I'd be looking deeper at the lamp and the zero cross circuitry.

Let us know what you find!