Abit NF7 board has high potiential to overclock, it might be your Barton cannot go to high frequency. Try to use higher Vcore and VDimm...also voltage up your NorthBridge

have you ever try to overclock your processor on other board? Like MSI, Asus, DFI or Epox. If other boards can achieve higher speed and more stable then it might be your board or Abit problem. If not most probably is not the board's fault. Check the RAM timing, PSU voltage see if it is stable when increase in voltage...etc