You haven't 8 cores, In real you have 4 cores and 8 threads (4 physic cores + 4 virtual cores named hyperthreading). I have got I7 920 D0 and I overcloked it at 3.7 ghz with 1.175 Volts get on P6X58D premium. Could you send us a CPU-Z pictures or post your bios settings
THX