I think that partly you may be running out of memory, try splitting the feed into two using the limit and limitstart parameters.
You can view a feed limited to 10 products like so:
https://www.fitnesssupportservices.com/index.php?option=com_googlebasexml&format;=xml&id;=4&limit;=10
If you do that you can see that the feed generates correctly, but then something generates a 500 server error. It may be a plugin or your template. Try turning on site error reporting, you should see an error message that gives more information about what is generating the error. If you report it here I can advise you on this.