As Cube bicycles are well made and employ fairly conventional geometry, so can therefore be expected to provide secure, reliable handling. Even today there is some dispute over the causes of shimmy in a bicycle, with lack of adequate stiffness somewhere in the system a strong candidate. I would start by checking wheel and headset bearings for play and then ensure that the front wheel in particular is correctly installed and the quick-release mechanism secure. You might also get the outlet from which you bought it to check the bike for potential manufacturing defects.
Beyond that, the complexity of the issue is such that it is impossible to give a definitive explanation. Tyre pressure may have a bearing and should be checked frequently. I have known a motorcycling ‘tankslapper’ (a similar effect to cycling’s shimmy) be provoked by a change of road surface while cornering. You might try to avoid ‘trail-braking’ – braking and steering together – as far as possible, but that is good advice whether your bike wobbles or not.
Many cyclists who have experienced a speed wobble report that pressing a knee against the top tube, or gripping it with both as you rise off the saddle (pictured), cures it. Knowing this solution may help you ride the bike with confidence. If not, your only option may be to change it.
Cycle’s Technical Editor