This sort of behavior shouldn't be surprising at all. When you change
the EFC (especially by a fairly large amount to move it a few Hz), you
change the (transient and steady-state) operating points of the
circuitry, so it has to drift gradually to stabilize at the new
conditions. The effects may be tiny, but so are the differences you're
looking for. Also, moving the frequency far away from "ideal" changes
the tempco, since it's no longer at the ideal center of the turnover
point. In reality, this may not matter much, since after all these
years, things may have drifted and aged way out of ideal-ness anyway.
The EFC bias (varactor leakage) current changes too, which interacts
with the external driving voltage source impedance. For lowest noise and
loading effects, keep the EFC driving resistance as low as possible.
Ed
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