Prime rounding error but no memtest error

Before you buy a new CPU and motherboard, loosen the timings (slightly) on your RAM and test again with Prime. It might work better that way.

Boot into BIOS, look at your boot settings and your sata mode and remember them, set everything to Optimized defaults (F5 on most boards), set your boot settings and sata mode to what they are right now, save and exit. make no other changes. Then run Prime again.

Watch your CPU temps very closely. If it gets too hot, shut it down. Before you stop the test, please post a screenshot (Screenshots and Files - Upload and Post in Seven Forums) of the Prime95 Window along with the CPU temp program you are using.

I switched to optimised defaults in the BIOS. I'm running Prime at the moment, with temperature about 40 to 49. Which is higher than before, but I did have the pc on for at least 10h. I got the same error (see screenshot) within 2min for Worker #1, but the others are still going. Previously I got that error on at least 2 of the workers (I really wish I'd made better notes).

I will try changing the RAM timings, but it might take 2 weeks before I can get back to you with the result.

leebeloola Your CPU is Hyperthreaded which means you actually have 2 cores, but there are 2 logical cores making a total of 4 threads. Prime95 is mainly a CPU stress test. It uses and tests some ram. The ram can and will make the test fail, but it is mainly a CPU stress test. You had 1 core fail which means 1 core and 1 logical core stopped. You will notice your screenshot. The main thread is a summary of the test, the workers are the cores and threads. thread 2 is not shown there. That, in itself, means nothing I don't believe except the layout of the window. In essence Worker 1 and 2 failed, core 3 and 4 continued the test. The test is very sensitive to BIOS settings and a small change can affect the results.

as Mellon Head explained, ram or ram timings can affect the test as well. I also agree with him that I would not buy a new CPU and Motherboard quite yet. It is quite rare for a CPU to fail unless it has been seriously abused, and usually over a long period of time.

What I would like to see is if you don't have it, please download CPUz (CPU-Z | Softwares | CPUID) and post screenshots of the CPU, Mainboard, Memory and SPD tabs. On the SPD tabs you can select each Dimm slot from a dropdown box in the upper left of the screen. Just post screenshots of the Dimm slots populated with ram. Please post the screenshots here, this will tell you how.
Screenshots and Files - Upload and Post in Seven Forums

OK, I'd like you to once more go into BIOS. Tell me the values of the +12V, +5V and +3.3V. I would also like you to see what your sata controller is set to (IDE,AHCI or Raid) and remember it. Look at your Boot options and see how they are set. Next, please set everything to Optimized Defaults (F5 on most boards) then make sure your sata controller is set to what it was when you looked at it. Check your boot options and set them the same as they were when you looked before and tell me the CPU voltage (Vcore) and Dram Voltage Save and exit, change nothing else.

Please open CPUz and open the CPU tab, open Core Temp and Prime95. Please Run Prime95 Blend test and let me see a screenshot of Prime, while it is running showing all 3 screens at the same time (Prime, CPUz and Core Temp). In the CPU tab of CPUz, please tell me the voltage ranges it goes through, basically high and low, as well as the max temps.

A 4.2 Overclock on a 2.9GHz 2 core CPU is a pretty healthy overclock. It would depend on how much voltage was necessary and how much it actually received. Obviously, if it begins to overheat during Prime, shut it down. Also, has it ever overheated in the past.

I'll preface this by saying that I'm not an Intel genius...

I'll add to the very good suggestions that essenbe made above. In addition to what he said, please also check the following in BIOS:

1. Check the status of the "EIST" parameter and set it to "Disabled" if it's on.
2. Make sure that "Load Line Calibration" is turned off. (The quickest way to screw up a CPU is to have it on)
3. Check that your DRAM voltage is 1.5V

If you don't have to do anything to the above settings, and after doing the things that essenbe said to do and it still crashes...

There are some things that you can try:

1. You can bump the Vcore voltage slightly (like 1 - 2 mV)
2. You can bump up the DRAM voltage 1 notch.
3. You can bump the Northbridge voltage 1 notch.

Getting to 4.2 GHz on that chip is a significant overclock. Now that you are running at stock speed, the CPU might be a little unstable from the stress of OCing it. It no doubt took a bit of extra voltage to sustain that OC, the CPU might have been slightly damaged and now requires a bit more Vcore to work properly. It does happen.

If I remember my Intel architecture correctly, core #1 is the physical core, and core #2 is the virtual core of core #1. It's similar for cores 3 & 4. What's happening here is that core #1 is having a rounding error, and is shutting down. Since core #2 is dependent on core #1, it also shuts down with a rounding error. This can be caused by three four factors:

1. Not enough Vcore
2. Not enough RAM voltage
3. The Northbridge running too fast
4. The Northbridge not having enough voltage for the speed it's running.

Given your OC, I would suspect the CPU Vcore first.

Try the things that essenbe said, and also what I said (if you can't get it running stable), and please post back the results so we can go further if we need to.

Great. We'll be here.