Bike chain slips in a strange way
You can see in the video how the teeth on the cassette rotate quickly at the moment it slips(at :05 seconds), while the spokes of the wheel actually stay slow, then jolt forwards to catch up. This means that the freewheel mechanism in the rear hub is failing to engage. 

It's actually quite dangerous because as it gets worse it could cause you to lose control, fall off etc. in the worst case. 

It shouldn't be like that on any bike, especially a brand new one. So take it back to the shop and have the freewheel, or freehub replaced (whichever type it is).