Larry,

Your mentioning a feedback coil made me think that your S-G might be 
a feedback design?  I hadn't thought it was, but In doing some 
feedback instruments, I have occasionally seen one thing that could 
possibly relate to what you are seeing.  If the loop starts 
oscillating at some relatively high frequency, sometimes the only 
noticeable effect will be that the instrument performance gets bad, 
and such oscillations can possibly come and go.

If you can find an oscilloscope to look at internal signals, it might 
be able to tell you something.

Regards,
Brett

At 09:21 PM 6/21/2012, you wrote:
>Well, I haven't tried locking the pendulum, but in an earlier 
>incarnation of this problem (in '03!) it tried a few experiments 
>where I disconnected the that supplies the power to the oscillator 
>and the connection to the feedback coil.  Monitored the output and 
>found that with the cables connected from the detectors I had 
>significant noise and with the cables disconnected still pretty much 
>the same.  But if I terminated the inputs with 2K resistors, the 
>output went to zero, save for a little +/- 1 count jitter.  So, at 
>that time I concluded that the board probably wasn't the source of the problem.
>
>But I find your suggestion of replacing that capacitor very 
>intriguing, and I will definitely try it.  I keep a zero center 
>meter connected to the leveling test point as an aid to getting good 
>centering.  And when I've been having problems, I have noticed some 
>jitter of the pointer around the set point.  So I'm very willing to 
>believe that the problem might be in the feedback circuit.  And, 
>it's a pretty easy experiment to try.  I'll let you know what happens.
>
>As fate would have it, the thing has been running pretty well all day today.
>
>Larry


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