Usually EPiServer CMS UI only reload the whole page when it is needed. That is when you change context to another page, block, media or a another version.

To update the preview of a page when you edit, only a part of the page is replaced by javascript manipulating the DOM. This can cause side effects so there are ways to trigger a full refresh when you edit certain properties. I guess it is not this issue you are experiencing? 

If you turn on EPiServers Content Caching it should not add any browser cache headers for content where WorkID>0. Actually it sets cachability to private and expires to yesterday in this case so nothing should be cached in your browser.

It would be intresting to see the http response headers for the request fetching the preview of the page (without you turning of caching in developer tools). For this to work you probably have to clear the cache first.

My guess it that other configuration or code is adding cache headers to these requests.

Thanks Fredrik.

True that the first thing is not my problem.

I totally agree that the problem seems to be that something else are setting the cache header but I cannot find out what.
The strange thing is that it does not always happen, right now I cannot reproduce it anymore.
I recored a video showing the problem that I send to support, if you want I can mail it to you if you would like to see the problem

I think I have found it. We were setting the response header Last-Modified to the time the page was saved and because of that the request to the page send the parameter If-Modified-Since and it seems like there is a miss in time there.

I have no removed the manually setting of Last-Modified and cant right now reproduce the error.