Sims Won't Move, Time Frozen, Can't Toggle to New Sim

I've been experiencing this problem for a few months now, where I go into my legacy save and it plays fine for a few minutes, but then suddenly the time stops advancing, my Sims stand still (but still move, swaying their arms, looking around, etc), and I can't toggle to a new Sim in my family. The Sim won't do any of the activities in their queue, they just stand there. Time doesn't move forward, and sometimes actually goes backwards by a minute or two. I can't switch to a new Sim. If it hit the menu button, it will switch to the Sim I last tried to switch to, but then I still can't do anything with that Sim. I've noticed this happens most often when I'm on an infant and then try to switch to another Sim, but that isn't the only time it happens.

I currently have UI Cheats installed, but I have removed everything else that I've previously played with. I have tried to play this completely unmodded, without UI Cheats, and it didn't make a difference. I have also created a new save with my legacy family, since that particular save file was getting old; that has not made a difference.

Any help would be appreciated!