Hasrrates and shares drops on exactly four hours to very low and starts returning to normal after that

Hello everybody, im pretty much nevbie in mining this is my second try after couple years. I have one problem that i didnt see first time.
My hash rates and shares drops down to a some minimum and then recovers from itself on exatcly 4 hours 10:30 14:30 etc… i ll put a screenshot of graphs if you can answer and help me. Rig works stable, no other issues, i m just wondering why is that… thanks in advance.