DPC Latency Peaks And Network Connection Losses

I've got a new PC which is now about 4-5 weeks old. It worked great in the first 2-3 weeks, but then the network connection started to be unstable. That means that the connection is lost regularly (in intervals between 15 seconds an several minutes). After it is lost there's an immediate reconnection attempt, which usually is successful. So the connection downtime is mostly inbetween 2-10 seconds. Now that does bother me pretty badly. Especially in online games this leads to many disconnects or lagspikes of up to 10 seconds.

The connection losses always come paired with DPC latency spikes.I switched the LAN cable just to be sure. I updated all drivers (including BIOS). I deactivated all eligible components in the device manager one by one. The only thing I found out: If I disable the network adapter, the latency spikes are gone. But that doesn't work because the connection is gone too. For that network adapter I tried different drivers, various versions from Shuttle and also the latest original driver by Realtek.