The network falls off?

Question

linux

The current version of archlinux on two PCs.
One-to-one updates coincide (the second PC was poured from the backup of the first).
The first PC is connected to the router via cable and works fine.
The second PC at some point stopped working normally through the same router.
Both wifi and cable. ping shows a break. Those. conditionally 5 packets pass, 100 - no. At the same moment everything is fine on the first PC. Then again 5 passes, 100 does not pass (the numbers are not exact, but the ratio is observed).
Reboot doesn't help. The second PC connected to another wifi point does not show any problems (ping is stable, without loss).
iptables is empty, firewall/qos is not enabled in the router config, etc. and the settings haven't changed in a long time.

Answer 1

rarely, but it happens (I once encountered it): check for collisions / matches of the MAC addresses of network devices on PC1 and PC2 (by ifconfig).