Visual / Code Editor alters my html

Hi I’m using FireFox 2 on XP (though I can replicate this in IE7). When I add some content into the visual editor and save it, it displays fine. However, when I come back and alter the content it changes the HTML, for example I added this content intitially:

<div class=”thecourse”>
<h4>Valley – Hole 1</h4>
<p>You need a good drive to the marker post. This leaves the green open but go left and you are in trouble. This is a difficult par 4 to start and a 5 is good.</p>
</div>

When I come to edit, it changes it to:

<p class=”thecourse”>
<h4>Valley – Hole 1</h4>
You need a good drive to the marker post. This leaves the green open but go left and you are in trouble. This is a difficult par 4 to start and a 5 is good.

So as you can see it strips out the div tag and changes it to a p.

Also when I added this content:

<table id=”course” summary=”Hole 17″ border=”0″ cellpadding=”7″ cellspacing=”0″>
<tr valign=”top”>
<th>heading</th>
</tr>
<tr valign=”top”>
<td>Fairly straightforward but be careful of out of bounds all down the right of fairway!</td>
</tr>
</table>

When I came back to edit, it took out the middle tr, so it ends up like this:

<table id=”course” summary=”Hole 17″ border=”0″ cellpadding=”7″ cellspacing=”0″>
<tr valign=”top”>
<th>Heading</th>
</tr>
</table>

I’m pretty new to WordPress so any help would be appreciated.
Thx,
Darren