need to no wot will cause this?

hi, i have a 1.0L vw polo 1997,
when i start my car there is no problem its not till it get to the right temp, all of a sudden my speedo keeps going up and down 0 to 60 back to 0 then to 30 never stays still, then my car will jerk all of a sudden all my electrics stay on no change in head lights nothing like that just that jerk every couple of min's while i maintain speed and my speedo going mad please help
thanks

AJ
June 2009