We have a Magento 1.9 installation, which we've moved from one to another server. We did an update from 1.7.2 to 1.9. Since we did that, the frontend sessions seem to get cleared within a time span of 5 to 30 minutes, occasionally. There is no problem in the backend tho. I just get forwarded to the login page.
I've checked all forms, if they have the formkey included. The sessions are stored in files, not DB. The cookies do look fine, as we have multiple perfectly working installations of Magento 1.9 on the same server. The Domain has not changed and it is secured by a valid SSL certificate.
We don't have any more clues to check, analyze or even solve this problem anymore.