Linux display is unstable and only semi-stable at 60Hz. What should I check?

Check whether an older kernel is still installed and try booting into it from your boot menu. If the display works normally there, that points toward a kernel-related regression rather than the refresh rate alone.

You don't necessarily have to stay on that kernel forever. A future update might fix the issue, so you can test newer kernels as they become available while keeping the working one installed.