MOTU 828 mkII always drops out

Hi guys,

I need help with my MOTU 828 mkII USB on Windows 8.1.

Audio drops out after like 3 minutes. I don't hear anything anymore, but in the windows control panel I can see that it's still outputting sound.
While the interface is connected to the PC, it is normal that you can't change the sample rate on the interface itself but only in the MOTU audio console software on the PC. Once the connection cable is released, you can change the sample rate on the interface. I just noticed that after the sound drops out I can change the sample rate on the interface. So apparently the USB connection gets interrupted. What can I do?

I tried the following things:
- reinstalled driver and tried older drivers
- tested the interface with other USB slots
- tested a different cable
- turned off the USB power saving function
- set the interface to factory default
- turned off the USB Legacy function in BIOS


My system:
Win 8.1
i7-5820k
ASUS x99 Extreme4
16 GB RAM Crucial Sports
Samsung 150 GB SSD
Seagate 2 TB HDD
MOTU 828 mkII USB