The most common source of the progressive brightening problem is a flaky connector cable. This problem was widespread in the 1xx 'book series. If the problem goes away when you flex or squeeze the frame of the display (try the left or right edge), that pretty much confirms the diagnosis.

The fix is easy. Disassemble the display portion, remove the flex cable, then reinsert it. Do this a couple of times, then test. Do not touch the exposed conductors (for the display's sake, not yours).