Every ten minutes Google Talk disconnects. The problem is best illustrated on the attached diagram. At first I thought it might have been ECMP load balancing but now I am not so sure any more. The reason why I don’t think it’s ECMP is because last time I check ECMP has a much more “regular” ‘refresh’ cycle. Not sure exactly what it is, but my thought is this problem is not related to ECMP. Does anyone have a clue?

If you have ECMP and NAT then you have a problem.
Thanks you Mr. Z. Are you inferring that this might be an ECMP problem?
Yes, that is why for such setups we developed PCC.
http://wiki.mikrotik.com/wiki/PCC