Firstly, using a combination of GTMetrix and W3 Total Cache I've got things set up to run really well (with the exception of minifying CSS seems to break some of the layout, but that's for a different post).
I just have one issue where I get a D for "Specify a cache validator" in Pagespeed, even though I have the "Set entity tag (eTag)" setting checked in W3.
The error is "The following resources are missing a cache validator. Resources that do not specify a cache validator cannot be refreshed efficiently. Specify a Last-Modified or ETag header to enable cache validation for the following resources"
Any ideas how I can get this to a an A rating?
Thanks in advance for any help.