3.3v rail randomly going low after a period of use

I am having a recent problem with my PC.

After a "random" amount of 100% stable running time (anything between 1 - 12 hours), the 3.3v rail will suddenly start dropping step-by-step from 3.36v to approx 2.8-2.5v over a period of 2-4 minutes. The machine then simply resets and reboots - but is then "good to go" for another random amount of time.

Although I know software monitors/loggers are not 100% reliable, I have logged the voltages using Asus AISuite, HWMonitor and HwiNFO and all show the same drop on the 3.3v rail just prior to the reset.

The 12v and 5v rails are rock solid throught the entire log - even when the 3.3v rail is falling. The tempratures of case, mobo, CPU and GPU are all well within limits. The fans seem to be doing their jobs.

Apart from the 3.3v drop everything else seems to be fine - even during the "final minutes"!.

This can occur at any time .... while simply using a simple text editor or running Grand Theft Auto V (as I did yesterday for 3+ hours without incident - then back to desktop, 30 mins later "reset")

If anyone can give me help or advice, I would be very greatful.

TIA

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MB : Asus P8Z68-V PRO GEN3 (LGA1155) CPU: Intel Core i7 2600 @ 3.40GHz (no overclock) RAM: 12GB Corsair Venegence DDR3 (no overclock) PSU: !000W Coolermaster "Silent-Pro" SSD: 256GB SanDisk SDSSDXPS240G SSD: 128GB OCZ-VERTEX3 HDD: 2TB Hitachi HDD HDS723020BLA642 GPU: NVIDIA/EVGA GeForce 4GB GTX 970 (factory overclock)

All other peripherals KB/Mouse/WebCam/2xHDD are connected to an exteally powered Anker 60W 7-Port USB 3.0 Data Hub

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