If your core is a good stepping you should be able to reach 1400 without any problems.
Heat can cause problems. What are you cpu temps??
voltage can help. But I've found that my cpu which is a 1 gig tbird, will do 1400 at pretty close to default voltage. The cpu will boot into windows at 1500 but it's not stable even at 1.85 volts.
Clearly, atleast for my cpu, most of the overclocking doesn't need a massive voltage boost. Once you hit the wall of your cpu, then you'll need more volts to push it futher. But it's a battle of diminishing returns. Volts=heat!
Hand picking your cpu is good! The store i went to allowed me to choose from a tray of cpus. So, i was happy when i found my axia Y chip. It's a great overclocker.
try looking for stepping here,in the middle on the die stepping for your chip will probably be somethin like AXIA or AYHJA prolly the latter and then there are other factors such as production week and other arcane letters or numbers might be present, sacrificing a chicken might also be a good idea. J