Need explanation for weird O/C errors

So this is my first time trying to o/c a CPU. My system - C2D E6420@2.13ghz, 3GB DDR2-800, WinXP, Asus P5K, 650W PSU. Asus Silent Knight II HSF.

Following general advice on the web, I first set the CPU multplier to 8x, and FSB to 325. RAM settings were left on AUTO. Managed to boot successfully, and OCCT reported system was stable at 2.6ghz. My CPU is reportedly able to hit 3ghz easily on stock vcore, so I tried that next. That was a nightmare. Spent days trying different settings of vcore and vdimm, but either couldnt' POST or kept getting core errors on OCCT.

This is what I did - first changed FSB to 375. System refused to POST with RAM settings on AUTO. My BIOS does not have a RAM multiplier setting, so I selected RAM frequency manually at 751 from a drop down list. System was now POSTing. However, OCCT and Prime95 reports error on core #0, and sometimes on core#1. Tried increasing vcore in small increments, but still got errors even up to 1.5v. Increased vdimm in small increments up to 2.0v and still got errors as well. I just about gave up trying to get to 3ghz. Temperatures were fine - CPU never got higher than 60 deg C.

Finally in desperation, I tried FSB of 400 with RAM frequency of 800. Voila! All stress tests passed and I reach 3.2ghz without any difficulty. I don't understand it? All the guides talk about overclocking the FSB in small steps, but my system only accepted an FSB of 325 and 400. Anything in between would give errors in stress testing, or would not POST.

If you overclock the memory (auto) too far, your overclock will fail even if the FSB is acceptable. Do not use "auto" settings while overclocking. Check the effective memory speed with CPU-Z if you can get into Windows. Do not use "auto" voltages either if you can avoid them.
Try reading this guide. Stress test with Prime95 for 24 hours your system.

HOWTO: Overclock C2Q (Quads) and C2D (Duals) - Guide v1.6.1

http://www.tomshardware.com/forum/ [...] uals-guide