Ford Focus intermittent wiper problem?

Occasionally when using the single wipe function of my 2003 Focus 2.0 Ghia, the wiper blades stop when I let go of the stalk (i.e. they stop part-way across the screen). This doesn't happen all of the time, & often cures itself for short periods when switching between this & the normal intermittent or permanent wipe.
I guess this may be a warranty repair anyway, but wonder if anyone has an idea what may be causing it?