You may want to try adjusting the "CPU PLL Voltage" in bios
On P5K Deluxe it goes from the stock value of 1.5V to 1.8V and on P5K values 1.5V & 1.7V can be selected.

My cpu achieves it maximum FSB with PLL Vcc at 1.910V.
Thats why the modification is needed.

Please note that on Asus boards PLL Vcc is tied with SB 1.5V Voltage, so raising it will also raise the heat dissipation on SB!