The network is not working well (periodically low upload \ upload + download) - where to dig?

Question

Computer networks

Client PCs - Intel Nuc, WIN10 Home OS, 2 pcs
Dell T30 Server, Win10 Pro OS, 1 pc
The whole thing is connected to the network via a Netis WF2780 router with a twisted pair cable.
User accounts are set up on the server, shared folders are made.
At first everything worked well, but the very next day there were problems:
Sometimes there are problems with opening sites, when measuring speed via https://yandex.ru/internet/ in such cases, the speed was shown extremely low either in both directions, or low upload with normal download.
Attempts to solve the problem due to the lack of knowledge-skills of network administration:
Insert-pull the twisted-pair connector from the PC back. It helped a couple of times, oddly enough (although the patch cords are new, not twisted / bent, factory-made) - problems with opening sites were solved. Those. The problem is clearly not from the ISP. And the router elsewhere was previously used without such problems.
Now I'm writing - the effect of presence, everything works.
Where in such cases to dig usually, what are the first steps in diagnostics?

Answer 1

Very similar to overloading the router, but not necessarily.
You can scan the network, look at open connections in the router, log (sniff) the traffic during the manifestation of the problem, with something like wireshark.