Does it only happen when the screen is good and wet? Mine had alot of slack in the system and if the blades where lubricated with good rain fall, they would move another 2-3cm when the cog reached it's end.
Try moving the arm (gently) when it's at rest. If there is alot of movement, it could be new wire, new wheel boxes, or just clamping down the wheel box tube to tighten the cable to the cog.