Peugeot 407 wiper blade out of sync.

My nearside wiper arm will sometimes sweep on its own,usually set off by opening a door, but not always.This arm, being under the offside arm,generally results in the nearside blade getting torn off---these blades are not cheap,plus the screen now has scratches on it.I contacted my local Peugeot dealer about this problem and they never got back to me.My son is a mechanic and he also is flummoxed,but he did lend me an extractor tool to remove the wiper arm,which is where I ,am now,monowipered.Has anyone mastered this problem, or am I alone?