Yes it definitely involves the digitizer as it is the only part of the display assembly that's responsible for sensing touch. Did you use 90% or higher isopropyl alcohol and a small soft bristle brush such as a toothbrush to clean the logic board and other parts such as the cable connections? If you did and the digitizer is still having the same problem then an area of the digitizer may have shorted out. If you are confident that the digitizers connection on the logic board and its cable end are clean and undamaged then replacing the digitizer should fix the problem.