Just posting this in case anyone else encounters this problem: I had a flickering screen on a Macbook Pro 8,2 often whenever I re-opened the lit and the screen turned back on. It however did subside back to normal after a minute or two/three. I was using the default open source driver. It seems switching to the proprietary fglrx driver fixed it.

If anybody else found another solution retaining the open-source driver, I'd be interested in hearing it.