FYI, the screen is resistive, not capacitive, so when you touch the screen with your fingers It can sometimes register unwanted multiple inputs, and since the N900 is not multitouch, only one point registers.

You best bet is to use your fingernail or stylus to drag and click, especially in the corners, where it seems less accturate (to me anyway) and don't forget to calibrate every so often. Hopefully you can get your problem fixed so you can continue to enjoy the N900.