Standard Asset Car/Flying scene Physics are really weird

As the title says, the Standard Asset Package from the Asset Store contains a few scenes that make use of Physics. None of them seems to work properly:
- the one where we fly a plane seems to 'stutter' really weirdly
- the one with the car looks like driving over a hill instead of a flat surface (it 'stutters' really bad, sometimes falls through the level and so on)

Used latest Unity Linux Build + Ubuntu 14.04