Zoneplayer 90 Dropping from group

Hi,

Quick recent problem...

My current setup consists of Sonos two groups.
One group has one device (zoneplayer 90) and the other has three (zoneplayer 90 and 2 Play 5s).

I've recently noticed that the zoneplayer90 in the bigger group occasionally leaves the grouping (with the Play5s) and I end up with three groups...

All devices are wired, and I'm using the latest firmware/software.

Has anybody else experienced this too?

My current (temporary) solution is to add the device back into the correct group, everytime it occurs. Which is a bit of a pain...

Cheers

Jonathan