Nvidia and rt-sources has a long-standing conflict. The current nvidia-drivers version that I can get working with rt-sources (currently 4.13.10_p3) is nvidia-drivers-381.22-r1. I am unable to compile any higher version than that against rt-sources.
If anyone develops a fix for this, please let me know, so I can incorporate it into Gentoo Studio and share it with other Gentoo users using rt-sources.
EDIT: See https://gentoostudio.org/?page_id=79, “I have an Nvidia card.”
It occurs to me that the nv and nouveau drivers cause xruns because they don’t dedicate video operation to the discrete card as well as the proprietary Nvidia drivers. There might be too much back-and-forth with Nvidia cards and the CPU with the open-source drivers, whereas nvidia-drivers does a better job of saying, “we’ll let the dedicated graphics hardware handle everything and not bother the CPU so much for video stuff.” I don’t know, maybe I’m wrong – please correct me if I am.