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

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 !