Like Inkwell mentioned: test your memory. The problem could also be a CPU issue. It may be overheating or even that the CPU needs more voltage and isn't stable at higher loads. Download
RealTemp and take a look at the CPU temps when running games or watching movies. With a medium overclock like that the temperatures shouldn't be getting far above ~75C-ish ever. Lower if the cooling setup is halfway decent.
Also, download
CPU-Z and check the voltage of the CPU. (It's labeled "Core Voltage") Note that when the computer is idle, depending on the BIOS power saving settings, the voltage and CPU speed may be lower than normal. Just start up a movie or game and check for the highest voltage.
If the temperatures look high and/or the CPU voltage is something absurd like >1.4V or <1.15ish, let us know.
To my knowledge, the Turbo Tech Feature will function regardless of clock speed. However, it will only increase if it has the spare overhead (heat and power) to remain stable and under the i5 Intel specified thermal limit, which is 72C. Personally, I disabled it on the i5 rig I had at 3.4GHz. I can't imagine it being really significant in most cases, and your overclock may already be faster than the clock speed "boost" from stock speeds.