Bob,
I saw something similar to this when setting up one of my ProgRock kits. After a series of writes, there seemed to be a somewhat random increase in the reference frequency register. Every read had it increase (though not by a constant amount) However, after power cycling I could not repeat the issue and that module is stable as others I have. I am not using serial control to try and turn it into a VFO, however. All of mine are being used in fixed output situations - set and leave alone.
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Paul KE7HR
Paul Jorgenson KE7HR