Phil, have you set the page that the user should be redirected to when trying to access restricted content? You need to set this to your page that you want the paypal buttons to be on? This is your membership options page.
s2Member is designed to prevent endless looping ( even if a site owner mis - configures things accidentally ). So, if you've somehow generated an endless loop, we need to have a look at find out how this was accomplished. Please provide as much detail as you can, about all of your configuration options that pertain to Access Restrictions. Also, please include a report that shows us which plugins you have installed along with s2Member. ~Thanks!