Rigid bodies pick up random movement and rotation (Accuracy)

[mav3]

I experimented with bullet and created a simple pendulum. It is a horizontal chain of three rigid bodies (1 by 1 by 0.1 plates), connected with point to point constraints to each other, and at one end above the ground. I used minimal code with default values. When I simulate it, the chain swings realistically, but the bodies pick up random rotations. After two swings, the plates are completely misaligned, and after six more, also the swinging direction has changed more than 90 degrees. The constraints of course do not prohibit the rotations, but is it normal that the movement is randomized so quickly?