Caster wheel "drives" the robot instead of following it.

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Hi everyone:

You may be familiar with my robot and its problems by now... here comes a new one, yay!

I think that I'm going to let the problem self explain with a video. As you can see, on ~0:30 I turn the robot 90º and then, when I tell the robot to go straight forward again, the caster wheel oscillates instead of just following the main body of the robot, making the robot move more like a snake than a car.

Hints?

Thanks a lot,
Miguel.

Update: I've noticed that if I add a lot of weight to the caster, the oscillation tends to stabilize, but I don't think that's a solution, because I'm increasing the weight of the caster wheel by 300 times its original weight (3g to 0.9kg). Any other ideas?