One question, is your lockring left-handed threaded? It should be and a proper fixed gear hub has two sets of threads on the fixed side, a set of inboard right-hand threads for the cog and an outer set of smaller diameter left-hand threads for the lockring. That way as the cog tries to unthread under back pedaling, it tightens the lockring and prevents just what you are experiencing.
A regular right-hand threaded lockring against a regular cog is going to be difficult to keep adequately tight as the torque that tries to unthread the cog also wants to unthread the lockring.
See Harris Cyclery's web site for a description of the function and configuration of these lockrings.