2004 Vauxhall Meriva jerking,lights dimmering and goin brig

Hi, i have a vauxhall meriva 04 reg which is runnig very poor the symptons are when i go to pulloff in the car it sometimes jerks trying to cut out and makes you almost bang your head althou it nevers does cut out, also the rev counter keep goin up and down, and also when stood in car at traffic lights or something with the head lights on they keep goin brighter and dimmer i have been told this could be the alternator but would it also cause the jerking and rev counter to bounce. There is no engine mangement light on had this checked and there is no stored codes! Any advice welcome cheers steve.