I drove my 320 CDI on the motorway yesterday. Set the cruise control for 70mph. Cruise control kicked in, but instead of maintaining a constant smooth speed (as it has always done), it sped up and slowed down by approximately 5-6 kph. I could feel the car surging and slowing, confirmed by watching the digital speedo display (in kph) varying between 107 and 113kph. When altering the speed up or down with the cruise control stalk the car continued to surge/slow, irrespective of the selected speed. Can anyone advise what might be causing this, please - and preferably offer a solution? Thanks.