The problem is caused by a voltage drop, commonly across the red channel of the tricolor LED. It's the first one to start getting dim. Once it gets low enough the game will throw the tilt, since the backlight has to be able to reproduce accurate colors depending on what the game presents as a winning combination. The major symbols have to show a solid color to correspond with the paytable.
The boards and reels themselves were made by Gamesman specifically for Advantage 5 cabinets, and for a while another manufacturer made them. There were two types in circulation, with different plug connectors, but they were both still 4 pin. When they first came out they were like 80 or 90 bucks a board, then eventually the price dropped to about 16. We used to get them from Patriot gaming or Suzo happ but they no longer list them. You may have some luck contacting them directly. Here's a link to the reel assembly:
http://www.gamesman.co.uk/gm5400.html