USB devices stop working with GPU load

As the title describes, I have pinned down an issue with my motherboard whenever there is a GPU load on the system. I assembled this system last week, and it hasn't been 100% stable. This is quite specific, though:

- I just got a Core i7-5960X with a MSI X99A SLI Plus motherboard, the CPU is running at a 3.7GHz overclock (from 3.0GHz) - There are **three USB controllers**, a standard X99 chipset Intel XHCI controller, a VIA VL805 USB 3.0 controller and an ASMedia ASM1142 USB 3.1 controller. - There are **two GPUs** in the system, a GTX 1070 and a GTX 970 (I believe both have a slight manufacturer overclock compared to their base specs) - The power supply is 750W

The **USB controllers behave as such**:

- **Intel controller is perfectly fine**, always works - The **ASMedia controller will die** a couple of minutes after boot (signified by any USB devices connected to it disconnecting) - On the **VIA controller, high-power devices will stop working when there is a GPU load**, and works fine otherwise. For example, my phone will stop charging, but my mouse will still work. - In the Linux kernel log, **only the ASMedia and VIA devices cause problems**.

I can think of several **causes** for this, but I hope someone out there knows better than me.

- **I could not get the ATX 12V rail to connect fully** (it was really hard to insert properly, almost feeling like it would break the PCB), it is about 95% of the way down its socket. If it is improperly connected, it might have a high resistance on one of the USB power delivery rails. - Maybe the **750W power supply** is not enough? - Maybe both the **ASMedia and VIA controllers are faulty**, or another motherboard component is faulty? (This doesn't seem very likely, but I don't have experience with this)

I have reproduced this behavior under Windows and Linux virtual machines (PCI passthrough), in both cases a GPU load is followed by high-power USB devices not responding until the GPU load is gone. This removes the suspicion of bad drivers in my opinion, since the USB controllers are detached from any host system drivers.

I have considered just leaving the two "broken" USB controllers behind and only using the Intel controller, but that would mean hooking up more external connectors (because the VIA and ASMedia make up 6 of the 10 USB ports on the back, and I need two distinct USB controllers for my purposes).