Performance unstable

Hi guys,
I noticed that the performances of the website are unstable since 15 days, that is what they change several times during the day, whether I made operations on the website (for example if i changing a small part of the text of a page or i insert a pic) or if i not touch the website.

I am attaching the screenshots made a distance of 1 hour of time.
http://prntscr.com/jydhvx Pingdom 23/06 ore 12.39
http://prntscr.com/jydib6 GTmetrix 23/06 ore 12.41
http://prntscr.com/jydipx pagespeed google 12.42
http://prntscr.com/jydxdr Gtmatrix 23/06 ore 13.30
http://prntscr.com/jydy4b Pingdom 23/06 ore 13.37
http://prntscr.com/jydyf6 Pagespeed Google 23/06 ore 13.38

Do you know why happened it? How can I solve it?

I love your plugin, because since I use the site it (about 2 months) performance has improved and before 15 days ago were stable and excellent.

I use the theme Enfold, I showed the problem to them and it seems that Gtmetrix, Pingdome and google developer, when they show bad performance it is as if your plugin was not activated.
Scan again a distance of time, it is as if the plugin is reactivated and the performances return good.

I noticed that if I hit “clear cache” for 5 or 6 times, the performances will return good for a while. Then come back worse again.

Can you help me to stabilize them please?

Thank you.

The page I need help with: [log in to see the link]