Does the screen look like this?
Check the cable that goes from the Main PCB to the LCD controller PCB and make sure it is not loose, check the solder connections on the jacks that the cable plugs into on both the Main and LCD controller board, the LCD controller PCB is very static sensitive so be very careful.Check for bad solder connections on the Main PCB and check all the electrolytics with an ESR meter, Look very carefully for bad solder connections on the LCD controller PCB, always use a magnifier to look for bad connections some times you can't see really bad ring cracks with your bare eyes.
There's a start for you......