wp-cron.php sometimes takes ages

Hi all,

Since a week or so our scheduled posts sometimes do not appear. I am researching why and all seems well on the server (a dedicated dual quad core, 16gb, almost no load when it happens).

When running wp-cron.php manually though, it sometimes takes ages to load. It loads eventually, but it probably could result in a timeout when run automatically.

The second time the page loads in a second or so.

Any clues what could be the problem? We use quite a lot of plugins, most notably w3tc.