TYPO3 Core

What is your exact PHP version? Possibly it's related to a PHP bug, see http://news.php.net/php.pecl.cvs/6605 and http://pecl.php.net/bugs/bug.php?id=8848

That you get the error only once a day might be related to caching.

I am using PHP 5.1.2 and also the APC cache, forgot to mention, so it is probably related to APC.

Will look if I can upgrade the APC cache.

I am running APC 3.0.12p2 - this seems the latest version, so I have to live with it in the moment.

You could try with the latest Debian package of the testing branch for PHP5: http://packages.debian.org/testing/web/php5

Or additionally could generally extend your /etc/apt/sources.list by debian packages of testing branch, so you don't have to wait for Ubuntu. But consider, that you're using possibly unstable releases then...

I had some minutes to dig into the c code of APC. It seems, that my previous suggestion won't help. So it seems to be only an APC problem. Could you please disable APC on your system to test if the error disappears? Thanks!
If so, it's not a TYPO3 concern and I'm going to close this bug-report.

Seems to be a PHP APC problem and not a general TYPO3 issue.
If the problem still exists after a patch for APC is available, you can reopen this bug-report.