We seriously can't know. Yes, using MP rendering might contribute to the issues, but beyond that we can't tell you anything. This sounds more like a systemic issue like the GPU overheating or some obscure tool kicking in in the background and gobbling up all resources and only monitoring with suitable tools can reveal the culprit. As a start, check your device manager and event viewer for warnings and broken drivers, then use your system's Resource Monitor to track performance and if even that doesn't help, use third-party tools...
-Checked the device manager: no warnings or broken drivers
-Checked the event viewer: no warnings, errors or anything critical in the last hour (in which I I had this issue again)
So I didn't manage to track down the issue...
BUT I decided to let it rest a few days (apparently computers might need vacation time after all) and then thought I should help the laptop breathe a little. Thank god it's built with steel because I can assure you that I can cook eggs and burn them with the heat that comes out of that vent area. Seriously, one cannot have its finger touch that spot for more than 3 seconds before getting real burns. Anyhow, point being that I've risen the laptop from my desk by putting 2 strips of wood on each end. Now there's enough room for heat to better circulate under. And guess what? I've worked 4 days on it without any of those buggy/crashy problems... Cannot guarantee that it actually helped, but I suggest for people to try this option if they're out of ideas.