I don't know if this is firmware, or slimserver, but I upgraded to 5.3 last 
night (including the firmware upgrades).  This morning, if I play an album from 
one squeezebox and then synchronize a second, they'll go about 2 or 3 songs, 
then one of the two (it varies) will experience a pause of 5 seconds or so and 
then resume playing.  At the end of the song, sometimes the two will continue 
on playing the next song, sometimes they'll both reboot, sometimes they'll act 
as if they're playing the next song but not report any progress (slimserver's 
fishbone skin reports that it is playing the song and that it is progressing).  
I'm having a party on Saturday and will be unveiling my new sound system - a 
great opportunity to show-off this hardware - and I need to resolve this by 
then.  FYI, this does not appear to happen if I play the two boxes 
independantly.