Hi,
This is the answer I got when I reported this issue to lkml.org (Direct URL to the report https://lkml.org/lkml/2014/6/30/77)
From Jörg Rödel and Borislav Petkov:
Hey Kelvin,
Most likely the BIOS is buggy and does not setup the IOMMU hardware
correctly so that it can't be used reliably in Linux. You can try to
update the BIOS the the latest version and see if it fixes the issue, if
not you should disable the IOMMU in the BIOS to get rid of the problem.
Or you pass 'amd_iommu=off' on the kernel command-line to disable the
AMD IOMMU driver in the Linux kernel.
---
There is no BIOS Update available for this Mother Board. It has the most
recent from Jan 2014. I couldn't find any option available in the BIOS
Menu to disable the IOMMU, very bad BIOS by the way.
So I tried yesterday a fresh install passing the 'amd_iommu=off' to the kernel
and everything is working fine. No delays/hangs, etc.
The workaround worked just perfect.
Thank you very much for your help!