Ladies and gentlemen I believe I have found the culprit.
Once I realized that the problem wasn't really that only some of the blogs were down, but that the whole site was actually down, I started thinking outside the box a little bit.
Sure I had been bumping up against QoS alerts for kmemsize, but since I doubled my memory a few days back those had been eliminated.
The problem was our good friend eAccellerator. When I disabled it and restarted the server I was able to write and edit posts, and the one or two blogs that were always down were up and running fine.
Apparently if I want to run eAccellerator I need to make sure to strip out kses.php. I've decided instead to go the APC route and see what dreams may come.
Thank you all for patiently reading through my woes and worries. :)