Think of everything that changed including subtle/weird things.

For instance, simply reseating the RAM or cards could have fixed an intermittent pin connection.

Opening the case to remove the cards might have been a factor.

Once I had a problem with a whole series of identical systems, they were all reporting "out of memory" dialog boxes in Windows when our company's software was run on those systems. When I took the CRT off the top of the computer's "pizza box" style case, the errors went away. It turned out that all the systems had been built with the IDE cables for the CD-ROM drive folded across the top of the drives and the cables were being crimped between the case top and the drives. (I'm not saying that's your situation, I'm just saying it could be something as simple as moving the case around could make a hardware problem go away).