Remove query strings from static resources plugins slowing down site

Just did a pingdom performance test for my site and got this error that keeps the site load slow. All the url with problems are from the different plugins, I use w3total cache but that does not seem to help.

Resources with a “?” in the URL are not cached by some proxy caching servers. Remove the query string and encode the parameters into the URL for the following resources:
http://domain.com/test1/wp-content/plugins/bbpress/templates/default/css/bbpress.css?ver=2.4

Got several of this urls, does anybody know how to solve this issue?

Hope to get some feedback