Q9300 Overclocking probs

Hey guys need some help. Is anyone else OCing a Q9xxx on a P43? I have a new Q9300 that I am trying to overclock on my EP43-UD3L. I'll list the detailed specs below. I had an E6300 (the 65nm Conroe not the 45nm Pentium) OCed to 2800 (7x400MHz) on this mobo, so I know it can AT LEAST do 400 FSB. The limit on the P43's FSB seems to be 420Mhz (could be a worse # w00t w00t). So i got the 9300 and Prime tested it at stock speeds (7.5x333) and volts, and it was prime stable for 8 hrs.

Here is where it gets annoying. Anything past 350Mhz FSB and Prime fails anywhere from secs to an hour. And anything past 375 and Prime causes the PC to lockup or the screen goes black within secs, and in either event i have to power cycle it via the power button. I've been running the RAM at 1:1 so it hasn't even exceeded 400 Mhz. The temp on the cores max out at 68C, and the temp on the NB heatsink (with a probe thermometer) maxes out around 51C, so I don't think its a heat issue. I've tried various voltage settings for the vcore, MCH, and dimms. The only way I was able to get it Prime stable while going past 350, was by upping the CPU termination to 1.5V and the cpu reference .995. But this seems awfully high to me, esp since those are the max settings in my BIOS. Is that too high, or is it fine?

Now i'm pretty sure the issue is defo the mobo. The reason I think this is that I had another 9300 which I exchanged for the current one. The reason I exchanged the original one for this one is that it too wouldn't OC stable past 350, so I was hoping it was just the CPU itself. Listed below is my specs:

Q9300 w/ Arctic Cooling Freezer 7 Pro
EP43-UD3L rev 1.1
4 x 1Gb OCZ SOE 6400 5-5-5-15 1.8V
Xfx 9800GT
Antec Basiq 500W