I wonder if the internal ADC connection is loose.
I had a 22" cinema display that was suffering from the same problem as you describe. I opened up the casing and tightened all the internal cable connections. That worked great for about a year and a half until the backlight gave out.
Opening up one of these is pretty straightforward, just make sure you keep track of all the screws. Also don't work on it with power applied. I know this usually goes without saying, but safety is always on my mind even though it does not always appear to be so.