Your wiper arm is in need lubrication. My 97 C230 was doing the exact same thing. The cover will need to be removed on the wiper arm and then just spray on some white lithium grease. I did it once 18 months ago and it's worked perfectly ever since. The problem was in the cold months (below 32 degrees f).

Make sure you have the wiper in the middle of the windscreen when you do it. The piston rod arm will be extended.