[Elecraft] K2 BFO drift problem

[TheMG .]

Well, a few weeks ago I finished assembling and testing my K2 S/N 7520 and
the KPA100, KSB2, K60XV, and K160RX options. All went without any problems
at and have had several successful QSO with it.

However, since then I've been noticing quite severe BFO drift. I have to
re-align the filters using CAL FIL almost daily. The low range of the BFO
keeps drifting upward, about 50-100Hz every day, and it hasn't stopped yet
even after two weeks!

Initially (first built and tested), the BFO low range (as measured with CAL
FCTR) was at 4912.6kHz. Today it's at 4913.35kHz.

What I've ruled out so far:

-Temperature. Room temperature is quite stable around 20-22C. Even after
long periods of transmitting with the KPA100 and the entire K2 chassis
getting quite a lot warmer than room temperature, BFO temp-related drift
was only about 30Hz and in the opposite direction of the daily BFO drift of
50-100Hz.

-At first I though maybe I hadn't properly secured L33 and that maybe
mechanical vibrations and stress from moving the K2 around was causing the
inductor to move around, but for the last week I had it sitting on a desk
without moving it at all and the BFO is still drifting.

Also, from my observations, the BFO range is actually constricting, rather
than simply shifting upward. The high range of the BFO has gone from
4917.47 to 4917.15.

Gently tapping or pressing on the RF board doesn't change anything.

Anyone experienced anything like this or having any idea what the problem
might be?