Cubase will stop recording if something inhibits its ability to continuously access the disk you are recording to. For example my backups are set to occur when I normally take lunch. But if I happen to be recording when my audio disk backup starts, the backup fully engages the disk for a moment which is enough to stop the recording. I assume something similar would happen if the CPU was fully used. I’d use Task Manager & Performance Monitor to see what’s going on when the problem occurs.