In the _wrapper.html Admin template, look for the line
<script type="text/javascript" src="$_weburl$javascript/loadWysiwyg.js" language="JavaScript1.2" onerror="errorLoadingScript(this)"></script>
On the default _wrapper.html, it's at Line 89.
Delete that line, and instead insert it at Line 56 so you now see:
<head>
<script type="text/javascript" src="$_weburl$javascript/loadWysiwyg.js" language="JavaScript1.2" onerror="errorLoadingScript(this)"></script>
<title>$settings.titlebar$</title>
This loads the WYSIWYG earlier, so when the function is called, it is (hopefully) always loaded.
This is NOT a good technical fix because it only works 90% of the time. I don't know enough about JavaScript to come up with a permanent fix. It is, after all, a race condition, and if someone at IT cares enough they can come up with a permanent fix.