yes thank you zax, for everybody with the same problem and I've fixed it and till now it works like a charme:

I've bought a "Tower Pro MG90s Micro Servo" - the one from tower pro seems to be the original (and I think you can also feel it, when you hold it in your hand, it feels like better quality compared all the other mg90s @ amazon) and now it works very nice.

zax also pointed out:
"the servo power (5V) is provided by the host PC USB port, so an MG90S current draw is marginal (700mA at stall). The SG90 is lower, around 200mA. Most USB ports are limited to 500mA although USB3.0 is typically 1500mA. Remember this also powers the Nano clone." so I've also bought an sg90, but tried the mg90s first -> i can not tell, if the sg90 with lower consumption would also have solved the problem