Vibration of the front wheel

Though I know the producer claims the pushchair is unsuitable for jogging, it happens with my new Freestyle 3 XL often that once you start to walk quickly (and of course when you start running), the front wheel starts to vibrate uncontrollably and the whole wheelchair likewise. Does anybody have an idea why? (Otherwise it is a very fine pushchair and this described problem can be "solved" by locking the wheel into a fixed position).
Thanks R. Prahl