To stop W3 Total Cache from minifying the feeds you need to go to:
- In the menu: Performance > Minify
- Meta Box: HTML & XML > HTML minify settings:
- Check the box for: Don’t minify feeds
One of the issues with feed minifying is that it adds CDATA wrapper around javascript in the content and that breaks the XML. I had this issue with videos that were in my content that used javascript.
Here is a screenshot that might help: