MacBook overheats and crashes

MacBook Retina 12 2017

• Just before end of warranty, laptop started crashing regularly
• Had the system board replaced (includes everything except the battery I believe)
• 3 months later, MacBook crashes when it gets too hot

If I push the MacBook, doing CPU intensive tasks, after 5-10 minutes it will crash.

I understand the laptop is passively cooled, and will shut down if it overheats. In the past, I've seen the CPU be throttled rather than a hard crash though. I have used it in a similar fashion in the past without issues. I would get a message about the system being too hot, CPU would slow, then it would cool down.

Is this normal behaviour? Is there anything I can do to improve the situation?

Obviously I can avoid running the machine at 100% CPU when possible, but sometimes that's unavoidable. My backup program uses quite a bit of CPU doing encryption for example, so working on the machine while that's running might cause this issue. I've also used an external fan, and then things are fine. Somewhat reduces the system's portability though! :-)