Vortex Sound Research

Hey guys, I've got a problem with my Novation Bass Station II for a while now. Everytime when I had it plugged in with a power supply after about 15 minutes it starts to fall out of tune. I know it is normal that an analogue synthesiser needs some time to get in the right tuning, but mine always falls out of tune after a couple of minutes plugged in. Did anyone of you have the same issue or knows a fix? Greetings from Germany