Andrea,
re: top - We cant run Top at the time of the crash -- the server becomes totally unresponsive and we have to reset the box.
re: timing - The timing is unpredictable. It had been doing this every couple/few weeks, and it wasn't a big deal, but now it's doing it every 1-2 days. Sometimes its less than 24 hours, sometimes its closer to 48.
I cant detect slowness before it crashes, but in one rare occurrence, I did see database connection errors.. about 30minutes/1hr later, the entire server was frozen. Could this mean the MySQL server is croaking first?
re: plugins - I have begin removing major plugins (like caching), and those which I dont need. I also went and upgraded every administration plugin I used. It hasnt helped so far.
re: anything new - The only new things that we have done are standard updates to WPMU, but there are chances I might be missing somethnig.
re: rss - any way to check for this? does this just create a major internal redirect problem? I dont have widgets enabled on the vast majority of my sites. There are only a few 'true' blogs that can use widgets like this.