I've been working on overclocking my i5 750, and so far I've run into some trouble. I'm at 162x20/162x21 turbo and I get crashes most of the time when I try to run a 3d application - BSOD screens indicate problems with nVIDIA drivers (I'm running a GTX275). Prime95 runs fine, as does memtest - which would rule heat and RAM issues out. I've just been working with 1.21v as my vcore up until this, so I'm wondering if I need to raise it to give more juice to the on die PCI-E controller. Is there any other answer that I've been missing? I pretty much haven't touched anything but the bus speed thus far.
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You shouldn't have to increase the vcore any more, at 3.6ghz I had it at 1.18V tho at 3.8ghz it needs 1.27. This is also with a much higher BCLK of 200. Your problem could very well be Turbo Boost. It does not go well with OCing. However, if you haven't touched your VTT voltage, that's the likely issue. I need 1.17V at 200x18 and 1.26 at 200x19. I tried putting my PCIe voltage controller thing from 900mV to 1000mV but I didn't really notice any difference when trying to OC the GPU so I don't think changing that will really do much but it shouldn't hurt anything, either.
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Also, another benchmarking app to test stability that I've recently been trying is called LinX. It's a little bit touchy to run it properly, but it's great because it gives you an output as to your computing speed, in giga flops. Check it out.