Problem, IMU freeze using motors

I want to build a selfbalancing robot.
Therefore I use two DC motor, the Arduino Uno, the Arduino motor shield and an IMU (MPU6050).
The IMU works fine aslong the motors are disconneted.
But when I connect the motors to the circuit, the IMU stops working.
I tried already to use pull-up resistors and also some decoupling capacitors.

Has anyone an idea what I can change to get the robot working?

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So there's a 6V battery powering the motor shield? And how is the Arduino powered?

Are the wires from the motors kept away from the IMU? I'd consider more decoupling on the 3V3
supply rail.