Don't power your geiger tube using usb with a laptop that has a GENERIC power adapter.

Welcome back Tine!

Yeah I've had that problem using the MyGeiger with the RPi and a phone SMPSU. The - on the secondary side is not way actually connected to earth so can float about. This can cause unintended 'detections'. I found the counter was only stable if I connected the coax screen and tube to earth. t think the problem was using coax for the connection between the counter and tube. I think the be st way is screened double core cable, with the tube connections on the internal wires and gnd on the screen. The only SMPSU with earthed negative I could find was sold out and unavailable, so that put the RPi setup out of action.

I only had the problem if the coax was used and the SMPSU was used. Removing the coax, using a linear supply or using the SMPSU with the - connected to earth solved the problem. In my case the extra counts were frequent so it was immediately noticable.