It was because of some really bad solder work of a previous person. Lifted pads, loose wires, weak or cold solder joints. One wire didn't had contact. I went over the mess, desoldered the socket, re-flowed all suspect solder joints, cleaned up everything, istalled 2 DIN connectors instead of 2 jacks and - voila - she works great and stable again, midi in and out works perfect. I also cleaned the potentiometers, just in case...