Hiya.
I'm having problems with the looping 5 second return notification. As far as I can see, IPN has been setup in PayPal (under Profile > My Selling Preferences > Instant payment notifications (update). I've trawled through the forum and noted a number of checks and tweaks but nothing has seemed to get this working.
Testing in SandBox was a real hassle because the PayPal SandBox wasn't working properly. So, with PayPal live, I put through a transaction on my credit card. My credit card has been collected, there is a IPN message in my IPN history but the return page looped on the notification page and there is no order listed in the backend and no email sent through with the download link.
I can send you login details for the site if that is the best way for you to check that I haven't missed anything.
Thanks!