I have just had my whole wiper system replaced cos i had all the problems going...juddering wipers and blades hitting the bonnet, i had knakered motor worn rack and tired arms.
which meant new motor and mechanism, new rack and wipers were converted to 145 arms with 1 19" blade and 1 21" blade. it all came to about £300 which was nice (not) but my wipers are now superb, quiet, smooth....come on rain bring it on.
Alfa aid in maidenhead did all the work and used only genuine parts.
hope that helps.