VPS running WordPress keeps crashing/stops responding

I have a WordPress blog which gets around 18K visits a day and its share of spam comments (attempted or blocked). Following an earlier project ([login to view URL]), some changes, including an upgrade in memory (from 256 to 512 MB) kept the server up without any problems for about 6 weeks. Since yesterday this week the problems are back and more. Virtuozzo shows CPU and Resource use at close to 100%, and lots of error messages: Black zone -- privvmpages. Looking for someone who can figure out what the source of the problem is -- the blog is pretty simply set up for a WP blog, with a few plugins and nothing very unusual. The VPS is hosted at SPRY, configuration can be seen here: [login to view URL] Given that the server keeps going down, this is as urgent as it gets. Thanks. Seb