After weeks of boring people to tears myself on this topic, I finally found a (so far) permanent cure without taking the motor to bits. After driving home in torrential rain, and running the wipers on 'fast' for about 10 minutes, the problem disappeared. I've since used intermittent regularly, and everything's been fine.
If this prob. still applies to you, try running the wipers for a bit on maximum, and let me know your results. How wierd, but at least the fix was cheap!