There's a connection issue with the tiny pins that connect your LCD to your motherboard. From experience, try to determine if you need a new motheboard or LCD if you can before you start buying replacement parts.
I'm having the same issue and this is my 2nd LCD/digitizer I bought off of Amazon. The issue is the connection from the LCD and motherboard; which is located right underneath the LG logo. In addition, after reading some other forums on this issue, there's some software issues when you install a non-LG LCD onto the phone.