Cache Lockout?

How can I turn off cache from within the database? I've tried emptying the cache table but still have the same problem on two different sites.

The problem is that a user can log in successfully, but any 2nd page viewed causes the user to be logged out. Hence why I'm unable to turn off caching from the admin/settings page.

I'm not sure why this started happening all of a sudden, but the exact same problem is happening with two different Drupal installs...

http://ridertech.com/
http://rowcoachmedia.com/

Any ideas? Searching for cache related issues returns many different results, but none seem to describe the above problem.