Reseating ribbon cables cleans them
Backbox ribbon headers are gold plated but the plugs usually are not; plugging them in and out a few times cleans the contacts and is safe, since they are rated for about 100 cycles. Don't do this to .156 or .093 in Molex connectors.
Systems
Source: Repairing Gottlieb System 3 Electronic Pinball Games from 1989 to 1996, Clay Harrell (pinrepair.com)