my guess is that the LCD cable is faulty or loose inside; perhaps the ground pin is detached, and the data lines are floating.

Once sign for this is that different screen patterns can make the problem worse according to the average voltage on the data lines which makes them float/drift to a different voltage and means the sync or clock will trigger or not

just a hunch.

I've seen this sort of behaviour on my zaurus 6000 when I don't connect the LCD cable correctly.

There are photo journals around of dismantling the 800, so I would carefully follow one and carefully clean any connectors and reinsert cables to try and cure this