Sounds like you are nearing the limit of the cpu, but I wouldn't give up yet. I had an e6300 in a P5B vanilla before this current setup, and it was stable at 490 FSB at 1.37v vcore. vFSB didn't help it at all, it just added heat, but your cpu may be different.

Try bumping up the vNB and vdimm a little to eliminate them as possibilities. I would use 1.26v vNB, 1.90v vdimm. Leave your timings at 5-5-5-15.

Also try and set vPLL to 1.52v, vFSB to 1.24v - 1.26v

Also bump up the vcore 1 step.

Try and see if that gives you better stability.

I have also seen gains in e6300's by adjusting the GTL values, i would try raising them by single steps to see if they may help you.