If you have problems with mod_gnutls since Squeeze. This might confirm something, at least if you use the multithreaded mpm-worker.
3 reasons for not using gnutls :
1 - No active development, same version for Squeeze, Wheezy and Sid
2 - Current version create a segmentation fault with apache server on Squeeze
3 - Just not compatible with apache2-mpm-worker on Squeeze
Will break your SSL websites and pull your hairs off!
I just had to move back to mod_ssl
You need Apache2 >= 2.2.12
You need OpenSSL >= 0.9.8g
Hope this is helpfull to someone.