Please help I have 404 errors I cant seeem to get rid of on wordpress affiliate sites

I have been having big problems recently with my affiliate sites dropping in the rankings for Google Yahoo, and MSN. I have discovered that the pages that are dropping all have one thing in common.

Their sitemaps are all showing a 404 error to a thank you page I removed a few weeks ago. This is still showing up in my google webmaster panel.

I have removed the thank you page on all of the sites and also resubmitted the site maps but I am still getting the errors.

What do I have to do to clean up these errors as I suspect it is what is killing my rankings?

I am using the Google site map plug in and I thought it automatically updated the site map whenever I added or deleted pages?

Below is a copy of that site map from the above site as it was downloaded from Google Webmaster tools 16 minutes ago:

<?xml version="1.0" encoding="UTF-8"?><?xml-stylesheet type="text/xsl" href="http://panic-attacks-treatment.net/wp-content/plugins/google-sitemap-generator/sitemap.xsl"?><!-- generator="wordpress/2.7.1" --><!-- sitemap-generator-url="http://www.arnebrachhold.de" sitemap-generator-version="3.1.2" --><!-- generated-on="April 14, 2009 12:22 pm" --><!-- Debug: Total comment count: 0 --><urlset xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd" xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"> <url>
<loc>http://panic-attacks-treatment.net/</loc>
<lastmod>2009-04-14T17:21:58+00:00</lastmod>
<changefreq>daily</changefreq>
<priority>1.0</priority>
</url>
<!-- Debug: Start Postings --><!-- Debug: Priority report of postID 3: Comments: 0 of 0 = 0 points --> <url>
<loc>http://panic-attacks-treatment.net/panic-attacks-treatmentnet/</loc>
<lastmod>2009-04-14T17:21:58+00:00</lastmod>
<changefreq>monthly</changefreq>
<priority>0.2</priority>
</url>
<url>
<loc>http://panic-attacks-treatment.net/about/</loc>
<lastmod>2009-04-14T15:11:09+00:00</lastmod>
<changefreq>weekly</changefreq>
<priority>0.6</priority>
</url>
<!-- Debug: End Postings --><!-- Debug: Start Custom Pages --><!-- Debug: End Custom Pages --><!-- Debug: Start additional URLs --><!-- Debug: End additional URLs --></urlset>

I can see no reference to the thank you page anywhere. It is and has been deleted from my word press site and yet I still have the 404 error.

Why and how can I fix this?

Thanks