How can i fix my Vauxhall Vectra 1.6 LS?

Hi all, im having trouble with my vauxhall vectra 1.6 LS, (v) plate (1999-2000), when driving at a constant speed the car is very jerky so to speek, its almost like putting my foot on and off the accelorator really quickly making the car jump around, (if that makes any sence), if im acceloraing its fine, lacking a bit of power, it only happens when keeping a constant speed, my engine managment light is not on most of the time but maybee once every 3-4 weeks it comes on and the car is fine ??? almost like new ??? as soon as i turn the engine off its back to its usual sluggish jumpy self. ive had my local garage look at it with no joy, so i bought myself a quite expensive diagnostic tool which does not seem to communicate with the car ??? ive tried the tool on a friends car which is the same car and the tool works fine.
i am now at my witts end , can anyone help, im in kent area.


cheers