I have Samsung Spica I5700 that have capacitive touchscreen, and I used a touchscreen protector.
Recently, I've noticed that the touchscreen often produces erratic movements, for example, if I put my still finger on the screen, the "cursor" will shake vigorously (sometimes very far off).
This makes the touchscreen very unusable; countless of times I've accidentally flinged the page when scrolling, while touching the screen for small scrolling; countless of times, the screen registered scrolling as a zooming or unzooming; and worse, it often flings to the wrong direction from my swiping direction (indicating a random erratic sensing). Sometimes, when I tried to touch a button/link on the top-left, the screen will detect a click in some other far away button (i.e. one not under my finger). Any games that uses touch intensively also becomes unplayable.
Could this be caused by the screen protector? Since capacitive touchscreen is operated by soft surface of a finger (instead of a pointy stylus), is there any noticeable benefit of using screen protector? or are they just a waste of time and money? do you think peeling off the protector will have a chance of fixing those problems?