Peugeot 308 car shakes when started

Last month I purchased a used Peugeot 308 5 Door Hatchback 1.6 (2008 reg) with only 14,500 mileage. Service was done by the previous owner in July this year. It was all fine but for the last two weeks when I start the car, especially after a cold night, it starts shaking. The car shakes for a few seconds and the revs jump up to 1100 and down to 700. After a few seconds the engine is running fine again. Keeps revs at about 800 when stationary although. Why would it happen? Could it be that the car shakes when started because one of the spark plugs is not working properly? Or is it something else?