Is it possible that your HSF needs to be cleaned??? These OE fans run at high speeds to flow sufficient air and they can draw in a lot of dust.
Does your mobo BIOS have an adjustment for the fan speed? If so something may have caused it to reset to 100%. See if there is an option to lower the fan speed percentage and keep an eye on the core temp of the CPU to be sure you're not running above 55C under max load.
Building a reliable PC involves more than just assembling the parts. You need to be able to configure all of the BIOS settings appropriately. This can be quite involved and frustrating as it can require a lot of trial and error with stress testing. It is however often the only means to get a 100% reliable PC.