basically when im doing about 60 or 70 on the motorway in 5th gear the car sometimes kind of judders, if i depress the gas a bit more it gets worse so i have to slow down completely or put it in 4th. does anyone know what this might be? ive had the throttle body cleaned, new spark plugs, rotor arm, new rocker cover & gasket yet the problem still persists. can anyone shed some light? thanks