For what reason can the connection over the network disappear at the end of the working day?

Question

Network administration

Faced with a problem, at the end of the working day (+5-10 minutes) the network connection is lost (no access to the gateway on CentOS 7), some computers have no access to network resources. All this lasts about 15 - 20 minutes, then everything starts to work normally.
I will describe the general structure, there are 100 - 120 PCs in the network, Cisco 2960 switches (5 pcs) + 2 gateways (the first is connected directly to the provider, runs on CentOS 7, the second to the central office. Problems only with the first gateway, Internet resources pings, and internal there is no network) There are no
time limit settings anywhere.
Nobody faced a similar problem?