I have a 2005 Grand Scenic which has developed a problem with the front wipers. When I am travelling at high speed in heavy rain the wipers will work initially and then slow down and stop half way across the screen. After what seems like forever they will suddenly kick back in and start working again. I have tried to replicate the amount of water being thrown at the windscreen using a house pipe but the wipers work fine. The sensor also seems to be responding ok to water and changing the speed of the wipers accordingly. Any help would be appreciated as I am totally lost to what the problem could be.