WPMU randomly freezing OSX server(s) -- no errors

Alright -- this is a big one, and really driving me crazy.

My ENTIRE WPMU (2.8.4a) server is crashing, and the error logs show nothing -- no errors, no system errors. Just a locked up system that has to be hard reset. Up until a few days ago, it was happening about once every ~3 weeks. Now, its been every 1-2 days.

I know I'm not offering much, but I dont have much to go on, either.

We moved the entire WPMU instance onto a new identical box, and had the same issues (wanted to make sure it wasnt hardware related).

We are running on a Max OSX server with apache, and have used this setup for over a year with 0 issues until the past couple months.

I'm desperate and would love some suggestions for things to begin trying.

I already started removing any adminstrator plugins I didnt absolutely need, and even uninstalled some useful ones like super cache. The themes I created do not rely on any plugins for displaying content except vipers video quick tags and a contact form plugin used on only 2 sites (out of 50).

Traffic is pretty low, only about 4-5k every day across all sites.

I wish I had a trail of errors to follow, but the logs have been clean.

Thanks for any help you can offer!