It doesn't look like the power supply.
Your Corsair CS650M 650 W PSU got a total maximum power rating on it's 12v rail of 612W (489W @ 80%).
1070ti's maximum power consumption is 180W, multiplied by 3 = 540W (378W @ 70%).
You might be needing a new MotherBoard or SSD.
Problems in the Motherboard produces a lot of symptoms or some slowdown in the system, but overall, it's hard to detect.
What I'm suspecting is your SSD might be failing, freezing and some OS or Program related problems can be a symptom of a failing drive.
Tests you can do so far (one after another):
-Find a Hard Disk Drive for test and trials.
-Try your whole rig in a new Mobo.
-Upgrade to a bigger Power Supply. Preferably, 1000W for your 4 GPU.
1070 ti is 180 W at default, but all cards are running on 75% power, so that is 135 W (405 W total). If I understand it correctly, card always draws the power from the PCIE cable first, so if each 8 pin cable is rated at 150 W the cards shouldnt be even pulling any power through the risers, so there is still over 100 W reserve...as for the motherboard, I dont have any other motherboard, this is my only rig...and just buying new one for 150 usd without even knowing thats actually mobo causing this issues doesnt seem like a good idea to me. Is there any way to check that ssd is not behaving as it should?