Hi James1402.
This kind of thing happens when you are updating your screen faster than it can cope with.
LCD's like these are quite slow.
If this is a test sketch, add a delay() after writing to the display.
If not (hardly, as it is a Hello World sketch), only write new data to the screen and do not rewrite stuff that is already there.
If that doesn't help, or you aren't writing to your display continuously, try pressing gently on the black bezel.
If that helps, then you have a bad contact inside the display.
You could try to solve that by changing the clamps on the back of the PCB, that are supposed to hold the bezel on its place (rotate them a bit, but be careful).