Rain sensitive wipers

[gman76]

My friend has a 2000 (x) 306 with optical rain sensitive wipers, which take the stalk position of intermittent on lesser models, the wiper arms in this mode hesitate 5 times in the last 30 degrees or so in the down sweep. although when switched off in this mode the arms down sweep & park normally. When set in the "on" position the wipers sweep & park normally.

Anyone any ideas as to why this would be & what is at fault?

Thanks in advance.