If you have a desoldering gun, I'd have at it. Might want to let someone else do it if you don't . If you have the original ribbon cables in there, I would replace them. Reseating the cables won't help if they're bad and they can go bad. Might as well replace the caps while you're at it too, if you don't mind doing the work.
If it's not the cables then my next guess is the RAM. Should be the right one but ask Ed to confirm.
The traces on the boards can lift easily so remove all the solder and gently remove components. Don't force anything, components should almost fall out on their own.