I usually got 15k user each day on my site and a good dedicated server.
This componet have helped me alot and I have donated and im the lucky owner of a licence key and I recomend all the other users here to donate.

I use page Cache 1.08 and Sef Advance.
Should I turn of the "Use Cache" option in Sef Advance ? Its now on and the cahce time is 86400 seconds.

In Page cache I have set it to -1 and i use the database option. Http headers are recached at 3600.

Alomos every time I publish new content my whole website hangs, and I got problems accessing my site. I have to publish during the day, when the visits are low.
I see in phpmyadmin that the mysql database kind of reseting each day. sometimes the DB is 3/4Mb and then goes upwards to 16 mb.

caching is also on in global configuration.

any tips

I would use the Sef Advance cache... You are going to like 2.0... You can reset the cache without ever clearing it... What version of php are you using?

I guess 4, but how do I find it out for certain?

In the joomla config, go to System->System Info... Then look, the third line of display is your php version...

Database Version: 4.1.22-standard
PHP Version: 4.4.6