Why the ETAG always change of my D7 site
In my setup, if I am logged into the CMS, then the Etag seems to be written as a unix timestamp, which is clearly being done to force caching off.  So this may be the reason you see the Etag constantly changing.

However, even when not logged in, in my case the Etag seems to change when the associated page cache expires, even if the content in the resource doesn't change at all.  I don't know why this is happening.