I had an intermittent similar problem with my IBM laptop. Turned out
the surface mount connector on the motherboard with video had poor
solder connections. This connector fed the video to the lcd display
though the mylar cable. If you can run the thing with the case all
apart, very carefully prod and push on the cable and connector.

Also do not forget if the cable had an insulator on one side of it
where the bend is located, to make sure you did not lose it. One side
of those cables sometimes is not insulated and they put an insulator to
keep it from touching the grounded/conductive case.