Problem solved everyone. As I mentioned - the solenoid expander was fine. I even disconnected the solenoid expander and continued having the light flickering, which would trigger the solenoid expander when it was plugged in.
Traced the problem back to a wire on the aux lamp board that's in the backbox above the player 3 display. The third wire from the bottom, whenever jiggled, would recreate all the symptoms. I tightened/pushed it back down into the connector, all problems gone! Hope that bizarre/specific issue can help someone else if it ever pops up haha. Thank you all for responses anyway- motivated me to search until I found the issue.