Instructions on how to remove the older kernel lines from Ubuntu 18.04.
Upgrade to the latest kernel on Ubuntu 18.04
Instructions on how to remove the 4.4 and 4.8 kernels from your system.
Learn how to build your own kernel from source, and customize it to make it stable on Ryzen based CPUs.
This post is for reference and contains a youtube video from Roger H. Newell stepping through compiling your own kernel, with bookmarked timestamps so that you can skip to the point you need.