I’ve been experiencing what seem like similar freezes to you. Every now and then the screen freezes, except for the mouse. If I’m playing music that continues to play, so the Mac hasn’t fully frozen or crashed. I haven’t been able to pinpoint anything specific as the trigger, sometimes I’m in Safari, sometimes Finder, usually always when more than one window is on the visible Space, except that I’ve heard that WindowServer (the process that manages everything on the screen) has had some issues since High Sierra, the going theory is that it is as a result of the complete rewrite to be atop Metal 2 that High Sierra brought. That said, forcing WindowServer to restart (using SSH from another Mac or iOS device to get to the command-line of my iMac, which is still fully functional via SSH even when the screen freezes) doesn’t actually clear the problem for me. I can safely shutdown my iMac over SSH (sudo shutdown now), but then still need to force shut down my iMac using the power button because the screen stays frozen even after the command-line on the other machine indicates that it has shut down. At least this means I know that everything is safe regarding my files, as the iMac has been shut down normally prior to the force shut down. Upon restart everything is okay. I’ve gone at least a week or a week and half between instances of this happening, but it has still come back. I’m pretty sure this isn’t a hardware issue, because it started alongside High Sierra, which being that it brought Metal 2 means that it brought a total rebuilding of the graphics infrastructure in macOS. I’d really like to find a way to resolve this, but I guess if that is waiting for an update to macOS High Sierra that’ll have to do. I hope my experiences, though they don’t provide a fix, at least might provide some more insight, and certainly add another person that is having similar issues to the list here.