I've now noticed that it's worse if I pick up the laptop by one hand, holding it between my thumb and fingers of one hand, by a corner.
That implies that there's a physical component to the issue.
Right now, there's no flickering. Hasn't been for an hour or so, but it will start again at some point.
One wonders, will Apple correct this problem, or let us solve it for them?
I have been having the same problem on my Macbook Pro. As I develop websites, I found that this was very frustrating as it only appeared to happen sometimes and I could not track down why. Reading through this discussion I had an idea and tried making a few coding changes. What I found was if I included the code line <meta name="viewport" content="width=device-width,initial-scale=1" /> (in case this is not displayed as code correctly in the post it is, less than meta name="viewport" content="width=device-width,initial-scale=1" / greater than) on a page it sometimes flickers when I move to this page from another page but only when I was using Safari. Typically it flickers a few times and is then ok. By removing this line the flickering stops on my laptop. I am not sure if this is the same on all Macs and would love to know if anyone gets flickering on their Macs when using Safari. Please let me know if you see any flickering when viewing the following site
For the record I am using Safari Version 6.1.6 (7537.78.2) on a Macbook Pro with Mac OS X Version 10.7.5 Build 11G63b
I agree with the previous comments that this looks like a software bug in Safari that Apple need to address. As the line of code I removed is fairly standard it is possible that some of the sites others have had problems with also have the same or similar code line in the pages html code.
With some LUCK Apple will now be able to track down the problem and fix it.