Probably a loose connection. Can be on the connector to the mother board, then it is a matter of pulling it out cleaning the receptacle and re-seating the connector. In most cases this solves the problem. Make sure that all the contact plates on the receptacle are in good shape and none is bent. If bent, carefully put it back in position using the other plates as reference (do it very slowly and on stages to avoid snapping the plate).
It also could be a bad connection on the LCD screen, most likely power line, positive or negative is failing. Such a problem will typically require a lab to be fixed (soldering is tricky, temperature and moisture sensitive) or easier a new screen.
I would also suggest checking the LCD line to make sure it is not torn anywhere. If it is, very delicate resoldering can fix it, however, it is not necessarily an easy job.
Let us know what it is...