It's a Lachesis. Lately the pointer would kind of lag instead of move smoothly. Like when I move the pointer from one side to another of the monitor at one go it stops at 2 or 3 places for a fraction of a second. If I wipe the sensor with a tissue the problem is solved. But it happens again within 20-30 minutes! What could be happening? It makes no sense that the sensor is getting so dirty all the time, within such a small time-frame.