You can also increase threshold values at :
/proc/sys/net/ipv4/neigh/default/
there must be three files namely..
gc_thresh1
gc_thresh2
gc_thresh3
see their current value and try to raise it (multiple of
.. say if 128 then try 256 .
Though this is not the preferred, I feel there must be some misconfiguration or very large number of machines on netwrk.