I have a similar issue. Installed Ubuntu 18.04 on a Ryzen 7 2700x with
an old Geforce 8600 GTS. Using the built-in drivers, the video card
stalls (took me a long time to figure out it was the video card, after
going through all the Ryzen issues). I installed the nvidia-340 as
recommended, and
I have the same issue, but I do have a GeForce 8200 integral on the
motherboard. I was happily running 16.04 with nvidia-304 driver.
Upgraded to 18.04 and nvidia-340 (recommended driver) and the system
hangs a few minutes after booting. If I go into recovery mode and
remove nvidia-340, all good.
