It could be the LEDs. If you pop in a few regular bulbs see if they behave better.

I believe in attract mode the GI dims and brightens, and without GIOCD or something, the LEDs are going to not dim, but flick on and off.

This is normal in attract mode and this is exactly the reason nobody puts LEDs in the GI in Centaur.

what type board is it?
if it is wpc or that, try this: http://www.ledocd.com/

What happens when you press the flipper button in attrack mode?
It shoud go through the different objectives and talking about it.

Take out a few led's and put in normal light bulbs and see how they behave...
Bally did not made then dim in and out, but switches really quick.
It's controlled via an opto isolator (MOC3011) and the only way they could dim is via fast switching.
I also think it's just a problem with the led's.

GI_driver (resized).jpg