Sterling JP60 keeps going out of tune
I recently got my hands on a used Sterling JP60. This is my first foray into the world of floating bridges, so I am looking for some advice on a problem I’ve been having – while the guitar is in excellent condition otherwise, it currently seems to be pretty bad at being in tune. I did a few iterations of tuning the strings one by one, but after 15-20 minutes of playing, all or most of the strings go out of tune, even if I am not using the whammy bar too much. I have checked the locking nuts behind the headstock and they are tight.
Apart from taking it to a tech directly, is there anything simple I can check for beforehand or fix by myself?