Lose brightness control on Apple Studio Display

[Chris Lim]

I have stock Cube (apart from 80GB Seagate Barracuda 5400rpm) and 15" Apple
Studio Display, for a year and a half. In the last 2 months, the brightness
control has disappeared. Symptoms

1. keyboard does not activate brightness (F14 & F15)
2. left control button on display does not activate anything
3. Preference pane has no brightness slider

It happens like this. Brightness control is as normal. After a few days,
keyboard control gets flaky--brightness stays set to maximum even if you hit
F14.

A day or two later, brightness control is completely gone.

A load like itunes, long file transfers or games seems to trigger it. Once
it's lost though, it stays lost, and nothing brings it back. Already tried

Restart and reset open firmware

Delete systempreferences.plist

Switch OS, from 10.2.8 to 10.3

Switch hard drive to external start up

Any clues, anyone? Pleaase?