Take it apart and put it back together. Get hold of the pdf repair manual for assistance in so doing. That should cure any obvious loose connections, or rule out that as a possibility.

Apart from that, the early active matrix greyscale screens are not terribly reliable beasts. Your problems seem, however, to lie either in the circuitry controlling them, or in the ribbon connections and such that feed them and their controlling circuitry. A bad lcd is typically black at the edges after a few minutes, progressing to unreadability. The fact that you aren't reporting that is good, very good.

Soldier on!