Dell T3610 automatically switches monitor to standby mode - please help

Hello,

I bought two used workstations T3610, computers come from two different sources (from two different sellers)

First Dell T3610:
Xeon E5-1650v2 
32GB RAM
MSI GeForce 1060 Armor OCV1 6GB
SSD Samsung 850 EVO 250GB
HDD Hitachi Dekstar 2TB HDS723020BLA642
Dell U2715H
Windows 10 Pro

Second Dell T3610:
Xeon E5-1650v2
16GB RAM
GAINWARD GeForce 1060 6GB
SSD Intenso 250GB
HDD Seagate 1TB Constellation ST1000NM0033-9ZM
Dell P2414H
Windows 10 Pro

Both computers have the same problem. When I browse the web, the monitor suddenly goes into standby mode. I can't wake him up. I have to restart my computer. This is irregular. One day the problem occurs 5 times, another day 1 time. Could you please help me ?