I have an update on the problem.
When I first opened the case of the NS1, I noticed some brown grease on some of the soldering contacts so I cleaned this and afther this the problem got reduced a lot and like I said didn’t occur yesterday. Today it started again afther a long time so I took the bottom off again and by pressing the soldering where there is this greasy spot the oscillator became stable again. You can see the spot in the video below and the audio demonstrates what I ment with the oscillator being unstable until it recovered. Do you have any idea where this grease could come from? Oxidation?