renault megane 1.5 dci 2004 windscreen wipers only working sometimes?

My car has an intermittant fault with the front windscreen wipers. Getting gradually worse. Started off with stopping in the middle of the windscreen...i could get them going again if i moved the switch to rapid. However, now they wont work for hours at a time. I drove 150 miles in the rain with no windscreen wipers yesterday. Then this morning i tried them again and they worked. But not properly. Could this be a fault with the motor? Can it be rectified easily? Im due to drive back to work on Friday but i'm very aprihensive!