[Sven]

Originally Posted by Have FUN

I had this problem with my 2000 SG Standard, I removed strings and checked 12 screws and 6 nuts, the tuners were not fixed corectly.
Why? May be low frequencies from the amps or wood.... but now the SG stays in tune one week

This is not an insignificant point; I do this every time I get a "new" guitar, and periodically afterwards.

...and yes, the nut slots most likely need to be "refined" so that they work properly. You should also check the bridge saddle slots for burrs and/or sharp edges.