I had exactly the same problem, I agree with Sparcolite, nothing to do with the wiper motor or stalk, definately the control box. I also could not locate a new control box at the time, so I removed the existing one, I noticed tiny breaks in the solder lines. I resoldered over these breaks, re-establishing the connections, put the box back in place and the problem was immediately gone, intermittent wipers worked as normal agin, havn't had a problem since, that was over 3 years ago, hope this info helps!
Regards,
Henry