Asynchron playing in a group

Hi i have several stereo amp boards and several sub amps boards.
The stereo boards are mainly connected through LAN the Sub amps are connected through WIFi.
The problem appears on all stereo amps and subs independent on the position and combination.

If I group an stereo amp with sub amp it plays synchronized together but after several minutes the Sub plays asynchronous than he stoped playing and start synchronized and this repeats every few minutes.
It doesn’t matter if the Sub amp is far or near to the Wifi router or so.

Any hint?