If the ratio is 4:5 that is almost absolutely your RAM failing on you.
If you plan to go to 4 GHz or beyond you might need new RAM, but as you can see your current RAM works stable even at 960 MHz (3.46 was stable), so you can probably hit 4 GHz even on your current ram, maybe even 4.2-4.3.
Well, to get 3.6 etc. stable, what you have to do is go into your BIOS and find some was to change the "DRAM to FSB" ratio, if it's either "4:5" or "[Auto]", make sure you manually set it to "1:1". With luck that should make your 3.5 OC stable.
Edit: A bit of clarification:
At an FSB of 391, your CPU speed would be 391 * 9 = 3519 MHz
Since the memory ratio is 4:5, we have to divide the FSB by that to get the memory's base speed: 391 / (4/5) = ~489
To get the actual memory speed, the base speed is dual pumped, so you have to multiply it by 2: 489 * 2 = 978 MHz.
So at that speed your 800 MHz memory is running at 978 MHz.
Were you to lower the multiplier down to 1:1, the base memory speed would be 391 / (1/1) = 391, when dual pumped results in: 782 MHz.
So you would be running your 800 MHz memory at 782 MHz, which will be stable.