Severe Stuttering (processor overload?)!

Hello, all.
Has anybody ever experienced a sever stuttering problem with Cubase? It looks like the entire system is being overloaded, but the file worked perfectly well the last time I used it. Stranger still, the problem only occurs if I start playback after a certain point on the ruler. Before that point, playback is fine – even when the cursor moves into the problematic area. The problem only occurs when I begin playback from within the “doomed” area. Please look at the video at the link I’m including. It’s a screenshot video, so there isn’t any sound, but you should be able to see what’s going on well enough…

Stuttering Cubase

I’m using an iMac 27" running Catalina. For this playback video I’m not connected to any audio interface (using built-in audio), but I have run Cubase through built-in audio before without a hitch. Also, I just tried playing back through a USB audio interface and the problem still persists.