You need to use javascript to force the other frame to load.
Assuming you have all the rotation code in the sponsors page, you could do this in the contents page:
<body onLoad="top.right.location='some_page.php';">
So every time the content frame is loaded, it triggers a reload in the right frame. If you have problems with the browser caching the some_page.php, you can generate a random string, and append that to the URL of the onLoad function.
HTH
(Oh yeah, I didn't test the js since I hate the stuff. Here's a link that might come in handy too. http://www.pageresource.com/jscript/jframe2.htm )