Ok. I disabled the cache for joomsef.

If this doesnt work i will start to delete urls from it when it happens again, and try to identify the url that creates this problem.

i think it has something to do with some of my modules.
dont really know for sure wich one because the behavior is weird.
i suspect that its my poll (pollxt), but i dont know for sure.
i will come back with details.

As it turns out it was not from pollXT.
It was from the login module with rokbox.
I disabled it when the problem happened again and everything was fine.
So now i know for sure whats the problem.
But what can i do? Should i try to use something else?
Its really weird because like i said before it only happens after some time.
Maybe it creates some strange url, or something, that doesnt go well with joomsef, because , like i said, if i disable sef the site loads ok, but if i enable it it wont load (when i get that error) if i have the rokbox login module enabled.
So what can i do !?
Use something else right ?

You could do some debugging first - check the code of the login module to see what could be a potential problem.
Or try to compare URLs DB at the moment when it stopped to work and try to identify the URL that might be problematic.
However, this still does not sound like 500 Internal Server error, so for sure check your webserver logs if there is not any error message that you tell more.

Well i dont know what exactly u mean by not being a 500 Internal Server error but thats exactly what i get (and everyone else) if i keep this module enabled.
So if i dont get this in the next week or so, this is the sure cause.

The first thing you need to know about an "Internal Server Error" is that the error can only be resolved by fixes to the Web server software. It is not a client-side problem meaning that the problem is not with our plugin. This is a 'catch-all' error generated by the Web server. Basically something has gone wrong, but the server can not be more specific about the error condition in its response to the client. In addition to the 500 error notified back to the client, the Web server should generate some kind of internal error log which gives more details of what went wrong. It is up to the operators of the Web server site to locate and analyse the logs which should give further information about the error.