TinyMCE changes P into DIVs

Hi,
I’ve been building a site where I use two DIVs on almost every page – the first to place a semi-transparent png of varying sizes and the second within the first to place the article text itself.

This seems to work fine – but sometimes when editing or creating a new page tinyMCE creates DIVs instead of P-tags when hitting enter. This has an effect similar to a BR-tag and the html-code becomes filled up by these divs, which btw are copies of the div they were produced within. At least that is what it looks like when viewing the page… In the Visual mode it looks fine and in the html-view you see all these new DIVs created in the Visual mode.

here is an example of what it looks like (the entire page):

<div class="medium_kolumn">
<div class="medium_kolumn_text">
Here is the article text supposed to go.
Sometimes hitting enter here results in
<div class="medium_kolumn_text">being duplicated like this</div>

</div>
</div>

I hope my post and example is understandable and I really hope I can get some good advice. I have searched several forums and of course googled it. But I can’t seem to find any clear answers. Sometimes the problem or similar problems are blamed on the browser (Safari in my case, but I think it has happened while I’ve been using Firefox). In other places people have had fixes involving changing stuff in the tinyMCE-files, but I’ve had no luck there either. So help would be very much appreciated.
Thanks in advance!
/kalblo