I have done the following so far ..
Restored my phone through itunes
Reseted the phone
Restored and set up as new iphone
The only thing that so far has worked is downgrading
To ios 5.1.1 ,then everything works perfectly , by then upgrading to ios 6 the problem
Returns , so has to be an ios 6 bug as in 5.1.1 the auto brightness ect
Problem is sorted its an ios 6 bug , just watched cnets apple byte and they say the auto brightness does not work along with bluetooth problems , my only question is for apple why do you release firmware full of bugs , i expect better from apple , even today saw a black iphone 5 in the mobile shop and thought it was a dummy model until i pressed the on button , that phone looked cheap , so i guess my iphone 4s will need another years use , if apple finally get their act together and do something they did not do very well , test your firmware for bugs , i would rather have a delay than a useless iphone.
I have 4s with 6.1.3. I had 6.1.3 for many months and the problem started happening just a week ago. It happens 2-3 times a day now, randomly. I don't think I did anything to my phone lately, no new apps or other changes I can recall.
Has anybody upgraded to iOS 7? And if so, did the problem go away?
I've upgraded to iOS 7 and I have the issue on an iPhone 4S. I can't remember this happening on iOS 6, but there may have been one or two occassions where it did. So, with regards to Julia98765's post above, in my case it just got a whole lot worse with iOS 7, much as I love everything else about it!
The screen starts off on my normal brightness level (auto is off), and then anywhere between a few minutes to just a few seconds, the screen goes totally dark, like it has been slid to the minimal brightness setting. From then on, the screen fluctuates wildy, sometimes snapping back to full normal brightness, but mostly just dim flickering until it decides to settle down in its dim state, or if it has been locked for a few minutes by which time it is showing normal brightness levels again.
Then the whole thing repeats itself. This happens in one form or another pretty much every time I use my phone! The only thing I haven't tried yet is restoring the phone through iTunes so will give that a go and report back soon.
So, just updating my experience if anyone is still having the same problem.
Multiple restores through iTunes and multiple factory resets failed to solve the problem, and I didn't want to try the fabled freezer trick. In the end, I sent my iPhone back as the brightness issues were deemed to be most likely a fault with the backlight.
Ironically, the replacement phone was even more faulty than the one I sent back! Luckily, I swapped that one as well in-store!