Overclocking for long periods can be very harmful in case of temperature highs, even short ones.
You might have cooked up some part of the motherboard or some circuit inside that ultra-complex CPU (1.16 billion transistors).
The Core i7-2700k has the clock speed of 3.5 GHz, but thanks to
the Intel Turbo Boost Technology it can achieve the max turbo frequency of 3.9 GHz,
stopping turbo mode automatically when the temperature exceeds its limits.
Over-clocking is uncalled for when Turbo Boost can do a much safer job.
In addition, the 2700's are actually handicapped Xeons. When Intel makes a CPU by etching billions of transistors onto a silicon wafer it is inevitable that some may fail.
They get sorted out by testing, with the most perfect put in the Xeon bin for mission-critical purposes.
The deciding criteria on whether a chip is classified as Xeon, i7, i5 or i3,
is both the cache size and the highest electrical frequency it can tolerate.
Your 2700 is an i7, meaning that 100% of its cache (8MB) formed properly compared to 80% in an i5 (6MB).
Its core and graphics clock can reliably cycle 100MHz and 250MHz higher then an i5 respectively, but less than a Xeon.
This means that for your 2700, as the cache memory has fully passed the test,
it is on electrical frequency tolerance that your I7 has failed to achieve the highest grade.
I hope you have been keeping an eye on the running temperature and have installed
some software to alert you in case of a problem, such as SpeedFan,
and have verified that all the S.M.A.R.T sensors were functional
and have tested your hardware with IntelBurnTest.
To check out the hardware, I suggest replacing parts, starting with the CPU, then memory and so on. Test products that work by putting stress on the CPU and memory can be useful,
but most will only detect hard failures.
Some things to try in software :
In NVIDIA Control Panel --> Manage 3D Settings --> Global Settings, switch "Power Management Mode" to "Prefer Maximum Performance" rather than the default "Adaptive" setting.
Also about the OpenGL message "lost connection": You can find more information in this Microsoft page: Timeout Detection and Recovery of GPUs.
In short, the workload sent to the Graphics Card was greater than what the graphics card can process in the normal timeout of two seconds.
Two possible solutions to try would be:
- Reduce the graphics workload if possible, such as rendering to a lower resolution or with less detail.
- Increase the timeout to allow more time to complete the really tough rendering parts. Microsoft provides the information on how to modify the registry to achieve this (although it accepts no responsibility for it).