Could be your HDD, but in my experience with AMD cards, The Witcher 2 stuttered for me a lot with V-sync off. Try turning it on. The 5000 series is NOTORIOUS for uneven frametimes (the cards don't produce a smooth stream of frames, they sputter them out erratically).
Believe those who are seeking the truth, doubt those who find it. 2500K (4.2GHz), Gigabyte Z68A-D3H-B3, 8GB DDR3, EVGA GTX 570, Xonar DG, Fractal R3, Win7 64