The short: I'm having packet losses between my iMac and my router ca. 10-20%, but ONLY when using ethernet connection. (...thus with WIFI and the same iMac there are no packet losses at all)
The details: I can't say since when, but I noticed the packet loss ca. 2-3 month ago when playing a game. I tested it with Network Utility and only when using the ethernet cable I get a 10-20% packet loss when pining my own router. This packets is thus also there when connecting to the internet pinging any other website. But there are no packet losses with WIFI. I tried to use a Pc and MacBook on the same(!) ethernet cable and router and I got no packet loss at all. I also tested my smartphones and got no packet losses.
I tried to disable the iMac firewall, but that didn't help. To check the ethernet port on the iMac I tried to use a thunderbolt to ethernet adapter on my iMac and got the same 10-20% packet loss.
The technicals: I use a iMac Late 2015 4k and macOS 10.13.6 (High Sierra). My Internet is a HighSpeed 400 Mbit/s. I use no anti-virus or mac-"cleaning" software observing the network traffic.
Any ideas what is causing this packet loss and how to stop it ? Any malware and/or network settings that can can cause this ? Any idea what further tests I can do to identify the problem ?