Given the output, I'm suspecting the kernel DRM driver. Since it sounds like it was a recent regression, try booting earlier kernels (-18 or -19) and see if the issue goes away. If so, then this sounds like perhaps a kernel regression.
Otherwise, please attach a photo showing the garbage on the screen.
Given the output, I'm suspecting the kernel DRM driver. Since it sounds like it was a recent regression, try booting earlier kernels (-18 or -19) and see if the issue goes away. If so, then this sounds like perhaps a kernel regression.
Otherwise, please attach a photo showing the garbage on the screen.