wipers on 2000a 4

Hi, i've got a problem with my a4 avant from 2000. The windscreen wipers are acting very strange. when i push de lever once down (for one wipe) somethimes the wipers just keep on going several times.
sometimes they don't stop until i jiggle a little with the lever, sometimes they stop in the middle of the window.
the same problem acures when put in the interval mode. except that they don't stop in the middle of the window. could this be the relay? when the start wiping i hear a 'click' and when they stop i hear the same click. but when they keep on going i don't hear this click.
if this could be the problem of the relay, where could i replace this?
kind regards.
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