Skips while playing in sync

Hi!

I'm trying to make 2 players play in sync, but this generates a lot of skips during playback.

Here is my setup:

Server + client 1 :
- RaspberryPi
- Logitech Media Server 7.7.3
- Squeezelite
- Wired connection
- Name : Malinka, MAC : b8:27:eb:fb:29:21

Client 2 :
- RaspberryPi
- Squeezelite
- Wifi/Wired
- Name : Touchradio, MAC : ec:1a:59:0e:10:8e

When playing independantly, everything works great, both Rpi can handle the load easily.

But when I want the 2 Rpi to play in sync, the playback skips (on both client) very often (every ~30s - 2 minutes). Most of the time, the playback skips for 1s and the restarts, but sometimes, it totally stops.

I try to connect Client2 on wired connection without any improvements.
During playback, the load on the server is ~50% CPU, but when it skips, CPU load rises to 100% for 2-3s.

I activated the log player.sync on the server.
Here is the output when both player are sync'd but not playing :
...
And when they play, until a skip:
The skip occured on the line in red.

I really don't know where the issue could come from. RaspberryPi? Network? Clock issue? Squeezelite? LMS?

Any ideas?

Thanks!