Cal 1861 erratic Timegrapher reading, what can be the cause?

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Hi, i got this 1999 Speedmaster 1861 yesterday, gave it a full wind and after wearing it 16 hours + 8 hour dial up rest, it lost just 4 seconds.
But on the Timegrapher dial up 12 sec interval i get this ;

- 39 / - 7 / -27 / + 17 / + 17 / - 41 / - 1 ?? Any idea what is happening ? looks like Shakin' Steven.

My cal 861 Speedy reads fine and has a steady straight line so i think the machine is okay.
Thanks for your thoughts !

 
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Thats odd, I've not seen something like that before! The frequency of that though makes me think it is something to do with the escape wheel though, since it is roughly every ~20 beats. Could possibly be a spot with sticky oil, or crap, or something?