Why are packets disappearing on the network?

Question

Computer networks

Some time ago, problems began with the network, namely, on some machines, communication with the server began to disappear for a short time.
there are about 30 machines 3 of them are malfunctioning.
they are all connected to the same switch juniper ex2200
server centos 6.4 clients on debian.
addressing is static.
if you run a ping on one machine to the server, then everything goes well until the packets start to disappear. and so it works through time. the percentage of losses ranges from 7% to 80%
already the brain has broken.
checked the cable with a tester. everything is fine. even the switch rebooted
any ideas?

Answer 1

In addition to possible IP address conflicts (most likely), check the load on the switch. It's not that it's very likely, it's just easier to check. If that doesn't work, see if any thinker is doing ARP spoofing. There was such a case - one eccentric with the letter M terrorized the entire network until they were caught :)

Answer 2

If I understood you correctly, you have 3 computers, "pings" from which to the server do not suit you.

any ideas?

The main idea is to find out how these three problem computers differ from the rest. The practical applications of this idea are as follows:
1) Are problem clients in the same L2 domain (vlan) as regular clients?
2) How is the network structured in general? One L2 domain or is there routing? If there is only one L2 domain, does it use one ip prefix ("subnet") or several?
3) Compare arp tables (commands like arp or ip neighrbor , your choice may differ) on the problem client and on the normal client, pay special attention to the resolution of the IP addresses of the server (if it is in the same L2 domain) and the default gateway .
4) Compare the output of the ethtool command ( ethtool eth0, substitute your interface) on problem and normal clients.
5) Next, select two clients, problem and normal, connect the problem client to the switch port where the normal one was connected and vice versa. What changed?
Having received your answers to the questions point by point (the output of the commands is also useful), it will be possible to build more specific assumptions.