E4300 - 2,63 Ghz limit

[Eyes]

Hi, I have Asus P5LD2 SE motherboard (i945 chpset), C2D E4300 (Zalman CNPS9700 NT cooler), 2Gb DDR2Kingmax 667 Mhz.
I reached 2.63 Ghz (733 Mhz for RAM) and tested with Orthos for 13 hours. It's stable and the max. temp was 59 (a lot for Zalman, I think that it's not such a good cooler after all).
My problem is that the sistem becomes unstable if I overclock to 2.7Ghz, resets or jams in windows. I increased the Vcore values but I have the same problem. DDR divider is 1:1 so RAM can't be the problem (600Mhz for 300 FSB).
I bios I disabled C1, TM2, intel spped step functions, the USB controler. I used Auto for PCI Express (if I set manualy to 100 the system won't boot at all).
Does anybody have an IDEEA what to do? I read on forums that 2.9 or 3.0 Ghz are easally obtained.
I am not willing to change my hardware so please do not give me answers such as: "change MB", "change RAM" (which is not an issue).

Thank you.

[BUFF]

The obvious solution would be to change the mobo but you don't want to hear that.
& most forums disapprove of crossposting the same thread topic in different sections.