OC bios settings changed back to stock without me knowing !?

No I didn't try that.
The Bios version is F2 but the latest is F3 which the only changelog is "Solve press reset button cause system abnormal"
I don't think this would solve the problem. So I didn't update the Bios firmware

what do you mean by saying running cpu at oc is not a good thing ? Everyone is doing that, aren't they ?
I didn't buy a 2500k series just for doing some benchs and leave it at stock clocks.

I know that. It happended a few times when I was overclocking at wrong voltages. But that happens when the system is restarting.

Is there anyway I can prevent that ? Is there any option in BIOS ?

Then How I should know where is the problem ?

Why the system is reverting the settings while it has worked for 2 weeks without any problem? Also I did all the stability tests. There shouldn't be any reason to revert the stock values.

A little off topic but why do you have 3x 2GB on your board? I have a similar set up as you do except I wasn't able to get a stable 4.5Ghz at your voltage. I had to go over 1.3...I think 1.31V to get it to be fully stable. I know not all cpus are the same but I agree with the others here saying that's it is not fully stable.

Running 3 sticks lowers the performance of your ram by eliminating the dual channel of just using 2x2GB. Unless they are 3 different sticks that you just put in there and they are all running at the worst stick's timings. It just confused me is all

It would be under the memory tab in CPU -Z. If you run 2 sticks in slots 1&3 or 2&4 they should come up as dual channel and will run at so many Mhz and x-x-x-x timing (lower timings are better). If you put three sticks then you will lose dual channel performance. It won't be a large difference but it's there.

Edit: Actually, do you have the type of ram sticks you are using? Sandy bridge requires 1.5V ram and if you are using old DDR3 ram that were designed for 1.65V; it could be causing your motherboard to reset.

Ahhhh, wrong way. Sandy bridge wants the RAM to run at 1.5V. It doesn't like it to be that high. I'm guessing then that your ram was at the right voltage.

and yes put the ram back to standard.

The memory tab should have a drop down box on the left side that shows slots #1-4. You can select each slot individually and it should tell you it's frequency and timings and in the upper right it would say dual on the channel line if it was dual channel.

You shouldn't use the Prime95 default settings. Use the small fft's test to test the processor. Use large fft's and blend tests to test CPU and memory. If only one passes, you sort of know where you should start looking.

One reason that I like Gigabyte motherboards is that they detect a failed overclock, they will usually keep trying automatically to find a bootable configuration. Many motherboards will force you to clear the CMOS settings before you can boot again.

When you are overclocking, you also need to monitor your temperatures. If you are going to overclock, I recommend better than stock cooling.

Intel's max recommended voltage for the i5-K's is 1.52 volts, but I wouldn't go over about 1.40 volts.

I remind you again when you start prime95 cancel the test. Now go to the top of the prime95 screen you will see left to right test, edit, advanced, options, window , and help. Click on advanced look for round off checking in the drop down list. Make sure it is checked, if not click it and put a check mark there. If you don't do this it will not catch crashes on the individual cores. I find and most sites about O/C recommend a minimum 10 minute round of testing of small FFT's it stress's mainly the cpu as the data all fits into CPU cache. After you get it to pass this I usually run the default blend test 4 or so hours then the in place large FFT's the same amount of time if I get a clean run I go on to try it out gaming or on video encoding. As for the video encoder Google for Handbrake its a cool free tool. I recommend on Asus motherboards to manually set all the voltages such as v core, memory, NB, SB etc. as Asus tend to over-volt a bit more than usually is needed to get stable. I avoid auto-overcloking like the plague!

This is all if your memory is at stock settings if you are overclocking it too go back to stock memory settings until you get the processor settled first. I to am curious as to why you are running 2x3 memory configure that is normally only used on the triple channel socket 1366 X58 Intel series. It probably isn't a problem, but as pointed out you are loosing out on your boards normal dual channel memory capability. You need either 2, 2x2 or 4x2 in other words at least 1 stick in channel A, and 1 in channel B, or for all four slots used. As for your temps what are they? Not all chips like the so called normal temps or flat just won't clock stably high. I have a Q9450 that taps out at 3.2 I can torture it at 3.4, but its throwing flames out and not stable at that setting. :^)

Have fun and good luck. Unless you can afford to buy trays of processors and lots of motherboards it still is really a lot of luck involved.

Changing the multiplier is considered an overclock. Was it at 13.3 and you set it to 16? If your timings were lower like 7-8-7-21 and just set the multiplier to 16 you could easily have problems. If the timings are 9-9-9-24 it shouldn't be a problem.

After turning the 'Performance Enhance' setting to standard, the timings have also changed automatically from 9-9-9-24 to 11-11-11-29

@A_Real_Overclocker
I have run Small FFTs for 1 hour without any errors. And Large FFTs for 8 hours and 31 minutes. No errors.
I am going to do video encoding.

Ok I finished video encoding, also playing a game. No problem so far.

So can we say that the CPU is settled ?

Then about about that "The system has experienced boot failures because of overclocking or changes in voltage..." message that appeared two times before ? What was that all about ?