windscreen wiper

[lordbarwick]

hi guys, i'm having trouble with my windscreen wiper. its a 1997 C180 classic and has the single wiper. the wiper extends as it wipes the centre of the screen and retracts as it gets to the edge of the screen, but mine is not retracting quickly enough and is catching the 'frame' around the screen and one time got stuck in the frame leaving me with no wiper movement at 60 mph and raining.
i have removed the plastic cover guarding the chrome piston/rod/arm, cleaned it oil/greased it but its still not great. somtimes it retracts in time and sometimes it doesnt, it works better the wetter the windscreen it ????

anybody got any ideas ?

thanks
Andy