Have anyone seen this behavior before? where POS system with Windows XP embedded get droped from ePO and then re-appear on as N/A for the system name.
Two thoughts come to mind:
1) Make sure remote registry service is enabled
2) How was the agent installed on your POS? One by one install for each POS or One image applied to several POS systems.
If it's one image applied to several POS, this may be an issue with duplicate GUIDS - You can spot check a couple POS systems to confirm GUIDS are duplicates:
HKLM\software\wow6432node\network associates\epolicy orchestrator\agent\agent guid