In a nutshell, in the good old days I'd use my Spyder 3 to calibrate my screen (mac cinema display, driven by a now 4 generations old MBP). I'd check calibration weekly. It hardly budged.
Since going to a newer MBP (that will soon be 2 years old). Same screen, if I'm editing I find i'm having to calibrate it on a daily basis. Brightness varies wildly - from way dark to way light.
I know, in the real world it's a first world problem - it's just getting on my nerves!
Syder software is up-to-date.
Could it be the graphics card.
Could it be the 'new' port the screen pugs into.
Could it be the screen - 4 years old?
Guess it could be 'all of the above' but i've no way (i can think of) that I can prove it to be any one / a combination thereof? I've certainly no access to another screen...