Hi, Burak from your schematic and your explanation I understand that is just a power supply problem! Add 10nF capacitor to the Vdd pic pin. Since surely you are using battery, my suggestion is that you use a two 9 volts batteries in parallel to a 7805 and use its output as power supply for your project. One way to check quickly that current failure is your problem, can be achieved simply removing (disconnect it) the backligth of your lcd, and very likely the problem will disappear as long as the present battery will be able to supply the required energy