Screen problems like you describe could be due to the flex cable not making proper contact with the circuitry on the glass layers. This may or may not be easily fixed, depending upon how the cable is attached. Sometimes there's just a clamp that requires more pressure. But usually it's bonded with adhesive, and this may be impossible to deal with without specialized equipment. But if you are able to open up the device and still have it operable, sometimes you can just try squeezing on different parts and see how that affects it.