gazebo robots always drift to the side

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Hello,

I'm trying to cerate a two-wheeled robot that follows a line (see other Question). But when I spawn a urdf model and publish a /cmd_vel message with velocity.linear.x = 1 and all other values at 0, the robot rarely drives in a straight line. Most of the time it seems to misalign itself and differ from it's path. Seemingly random to which side. (https://ibb.co/S5m7H5C) Also, the rviz and gazebo positions don't always match up, which could have a comon cause?

I thought it may be a urdf error, but even with predefined models (e.g. the turtlebot) the same problem persists. Does someone know what may be the problem?

Thanks in advance

UPDATE: I tried on a seperate system to launch the pre-defined turtlebot3 model and, again, send a /cmd_vel msg to it. But instead of dirving with linear.x = 1 it spins randomly.

graphic card: NVIDIA GK107GL (Quadro K2000) Ubuntu 18.0.4.2 LTS ROS-melodic