Check you apache logs (access and maybe error) to see what is happening. Another option is httpfox.
- Install the HTTPFox addon in Firefox: https://addons.mozilla.org/en-US/firefox/addon/httpfox/ and restart firefox.
- Close all tabs/pages except this one so other traffic is not mixed in the overview you will see.
- Click the start-button in httpfox.
- Open the site/page, and see if the problem occurs.
Now if the problem does NOT occur, you can reload and retry. In the meantime you might want to clear the httpfox history to keep a good overview. One pageload can result in hundreds of items downloaded, and thus hundreds of items in the httpfox overview. So you better open a simple text-page or product detail page with not too many items in it.
When the problem occurs, you get to see what is happening, which message the browser gets back, like a redirect. I believe http errors and redirects have a different color.
Hopefully it makes clear what is happening, so you can figure out what causes your problem.