404 error on some pages.

I'm having a random 404 problem. I have a WPMU setup running for years now and now I have some pages giving me 404 errors on my main site, not all pages just some, like all blogs (blog page), about page, and help page.

I already tried to deactivate plugins, re-save permalinks, change back to standard htaccess, checked with the default theme and still no luck. Please help.