More information needed. What version of FOG are you running? The clients that “suddenly” freeze: Are they in a multicast tasking?
What OS is the Server?
Partclone leads me to think you’re somewhat newer, but I cannot help without this information.
That said, as @Quazz stated, maybe these devices have bad NICs or bad firmware on the NICs that trip for no apparent reason? As @Wayne-Workman suggested, due to the “random” point of the freezes please reseat your RAM on these problem systems just to see if this helps anything.