De-tunning or what?

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Original poster: "Charles Hobson by way of Terry Fritz <twftesla-at-uswest-dot-net>" <charles.a.hobson-at-btinternet-dot-com>

Hi all,
 
When I power up my coil (using a NST 10kV/60mA)  I get massive sparks into the
walls and ceiling which are only about a meter away. The room is not ideally
suited for this so I don't know the maximum possible spark length. Anyway,
after about 5 seconds of power application the sparks wane considerably and the
safety gap (15mm) takes over. I switch off power for several seconds and
re-apply it to get the same big sparks. As I  repeat this process the time for
the safety gap to take over reduces to about 1 seconds. If I then wait a minute
or so, the process repeats all over again starting at 5 seconds of power
application etc.
 
I thought it was my spark gap crudding up, so I replaced it with my standby
unit. Both are identical static 6 segment multi-gap units. Nothing changed.
 
I am not certain but it appears that the secondary coil is becoming de-tuned by
something which happens during operation and that the energy released in the
primary (100/sec.) is reflected back to the NST via two 2mH chokes. The
operating frequency is 109kHz. That is, primary and secondary coils
independently are pre-set to this frequency +/- about 500Hz. The coupling k is
0.19. 
 
I would be most grateful for any comments
 
Chuck/England