Fiskov, that's good to know. What's your CPU voltage? I was basing my assesment on this data; Ghigo's chip (same batch as yours) required more voltage to get a lower speed than those of similar voltages:
3838A824 3815mhz On 1.33 True 120 The Nemesis
3837A909 4050mhz On 1.33 Ek sup. Wb SmokeyTheBandit
3838A755 4116mhz On 1.33 Fuzion V1 Fallwind
3837B017 4015mhz On 1.33 True 120 NBF
3837A768 3800mhz 1.34 unkown Wb Ghigo

Supershanks, that thread is where I've collected most of this data from. There's a lot in there!