If some percussive maintenance got it working again, I'm gonna go out on a limb and say that something's loose. Go through the box and check every bloody little thing, even if it doesn't look like it even has a connector. Reseat everything in sight. If that doesn't fix it, THEN go for complicated things like caps and such.

Also, when you take out the RAM, make sure your hands are clean and wipe off the contacts with your fingers. That might just be the issue.