I've never thought to use Windex. I always assumed it would leave a sticky residue. I use ~90% rubbing alcohol, and I've found it works well. Most of the luck I had, I think, involved simply the reinsertion of the board multiple times, producing sufficient friction to clean the contacts. It's one of those things that mileage will vary on, and it seems unfortunately yours may not be fixable the same way.

Another 57x series motherboard would be an easy way to try things out, but I assume you would already have tried it if you had one.