On some good advice I shut down the machine and ran Memtest86+ on it. As I suspected there were failures in the tests, so I started pulling modules and testing one at a time to see if I could isolate it. I wasn’t able to reproduce the errors at all, even when installing both modules as before, so I’m not sure what to think. But I’m going to watch the system and see what happens. If it really was just a bad connection that reseating the RAM modules fixed then I’m not going to have to spend money to build up a new server. And that’s a Very Good Thing.