Finally I found a solution:
1. Replaced the power suppy up to a 9V 500 mA adaptor
2. I found out that the reset switch of my console was also modded. It´s called "switchless pause-mode". Pressing the reset switch a shorter time results in pause mode (you can then continue by pressing the fire button on the joystick). Pressing the reset switch a longer time results in the normal effect of this switch (e.g. starting some games).
Took me nearly a year to find out but now i´m happy.