Failure to mention RAM. (A Link to the ONE you bought would be nice)
Failure to mention motherboard (And therefore exact model and BIOS Flash Information)
Failure to mention Power supply (a PC turning off during use suggests the Power supply is unable to maintain voltages or amperages needed, in turn it suggests a defective or underwattage power supply)
Failure to mention other CMOS Settings you changed while overclocking.
Changing the Vcore is not the only thing you have to do when Overclocking.
What have you done with RAM Voltage, MCH Voltage, FSB Voltage and what settings did you disable in the other sections of the CMOS.
Have a look at this guide.
Core2Duo Overclocking Guide v1.1
IF we determine that the power supply is not the problem and IF you follow that guide and still are unstable, And IF you Flash BIOS to latest version, and if you run memtest86+ on the RAM and everything checks out at this point and you STILL have instibility problems, THEN RMA the motherboard.