If you have a server-side HTML cache, it should be disabled for dynamic content, like wiki pages. Otherwise, I don't see how a different version of the page would show, since the URL is almost exactly the same, and is not the correct URL to view old versions of the page.
Also, have you tried updating to one of the newer releases? This report says that all the relevant fixes were added to Gamma 6 (current release is Gamma 7), and your site still shows that Gamma 5 is installed.