Ok, an update. I think I have found the problem and may have rectified it.
This is the problem I had -
Now you may notice that even though I was maxing out my RAM, BF3 was taking up nowhere near what I expected it to even though it is the definite cause of the stuttering. It appears, after downloading RamMap, is that I had a Nonpaged Pool totaling 7.5GB.
Even coming out of BF3 my RAM usage was in the high 90%. Only a reboot would cure it.
Digging yet further took me round the houses and back to the AMD Graphics drivers. The fact that I was running 13.1 was a pain as it is the latest one. Reports are that it is a known problem and had been fixed in the 13.1 drivers.
I have now downloaded and am running the 13.2 beta, and fingers crossed it appears it has gone. I have not had the chance to play more than two maps in a row though so it may well appear again.
I believe it is just an unlucky combination of Win8, my motheboard (Sabertooth 990FX), and my 5870.
We shall see.