Punto selecta 60 sx wobbling / shaking

I've just bought an S plate punto 60 selecta, the 1.2 model. I've noticed that ocassionally wnen going at about 25 - 30 mph the car
wobbles noticeably from side to side in a see-saw motion, it stops as speed increases.
Can anyone tell me if this normal -- i.e, a characteristic of the auto transmission, or if not, if anyone else experienced this could you shed some light please.
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