Permalinks goes wrong after few days/weeks

Hello,

I’m using wordpress since version 1.0 came out but I can’t figure something out.
I use the %postname% permalink structure (I know for the performance reasons it’s not the best structure (but it is the most user friendly I find).
And that works fine, but after a few days sometimes weeks all of the post (like portfolio post and all that) goes to 404. All the pages keep working but none of the posts.
When I then resave the permalinks (with the same structure) it works again like normal)

Any idea what this can cause?
The website is: mindofmedia.eu

Jurgen