I've been seeing huge spikes in server load that have resulted in repeated crashes

I have installed a collection of WPMU plugins to manage my wordpress multisite (gapyearblogs.org). I think the only non-WPMU plugins I’m using are iThemes Security and W3TC.

In recent weeks, the site has shown sporadic peaks of server load, crashing the server. With my hosting provider, I’ve spent hours and hours doing everything from beefing up security to reducing and distributing all cron jobs, to simply making the site leaner and better optimized. But, these problems persist.

Do you all have any thoughts? Are there any known conflicts among the plugins on my site?

Thanks,

Robin