Server crashes since installing WPMU

I recently converted my wordpress blogs into MU. They run just fine for a few days and then the server suddenly becomes completely unresponsive and I have to do a hard reboot.

I haven't changed anything else on the system other than switching to MU and there are only two blogs in the MU install. Nothing shows up in the apache logs or system logs.

I'm not getting any messages about the system being out of swap space so I really don't know what's the problem.

At this point I'm pretty certain that I'm going to revert back to regular WP. Does anyone have ideas on what would cause it or how to track down the problem and fix it?