Firmware 7.1.1 from Dec 23, 2014

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I had firmware 7.1.1 installed on my master and slave units for my D400,
but yesterday did an update to the version from Dec 23, 2014, and there was
an immediate improvement in the synchronization between the master and the slave.

Just in case you have not updated to that subversion of the firmware.

Did you check the version numbers for the separate parts of the firmware before and after changing it?  I thought we (users of this forum) had concluded that in fact there was no difference between the original and Dec 2014 "version" of the firmware?