Update: The problem is solved.

Basically, there were several stability issues affecting each of these systems (some were more pronounced, obviously, on certain systems). These are fairly small systems, and it turns out that I had several things touching heaksinks (especially the northbridge) which was really slowing certain systems up. Others were getting random Error #132s. I had done everything like BIOS updates, MEMtests, etc. Still no luck (and still thinking the slow loading wasn't caused by the same issue as the 132s), I posted here about the loading problem. In the mean time, I had completely reorganized where cables and other things were inside the boxes (after checking that the cooling was up to snuff). This included taking them all apart and cleaning out the garbage thermal gel that came on the board (I did the processors as a matter of course while building the machines months ago) and replacing it with Arctic Ceramique (God, I love that stuff). I should have done this while building the machines, but I didn't even think about it.

The RAM is plenty high-grade enough (Kingston, in fact). This should never have happened, but I guess I got what I was asking for when I didn't take proper care the first time around.

Moral of the story - never underestimate how worked your northbridge can become from even a few minutes of MMO stress.