Hi Tim Guinness,
Thank you for reaching out to Microsoft Community.
When your Surface Pro 5thgen overheats, does it then turns off, runs slow/lag or the device will just be hot and still run normal?
Here are some steps you can try to troubleshoot the issue:
1: Run Surface Diagnostic Toolkit and check Windows Update
2: Update drivers and firmware
Check what is the current OS Build of your Windows version by going to Settings > System > About.
Go to Download driver and firmware for Surface, choose Surface Pro 5thgen then click Download on the next page.
Choose the .msi file that matches your current OS Build and click Next.
After downloading, run and install the .msi then Restart.
3: Run System File Checker Tool
Please check this page for the step in Using the System File Checker Tool in Windows 10
If this answer your question, we highly encourage you to inform our Community by clicking on Yes or marking my post as an answer. This will help other members who may have the same issue as you. Otherwise, don't hesitate to update us so we can further assist you.
Kind Regards,
Marrion Y.