Why Does My Friend’s PC Drop to 10 FPS After a While?

I'm having a frustrating issue with a PC I built for a friend who's new to gaming. After some initial good performance, the system drops to 10 FPS during gaming sessions. This usually happens within the first few hours, but it could occur later too. I've monitored the system and tried the usual fixes—reinstalled drivers, updated the chipset, and even did a thorough cleaning and repasting of the older 3070 GPU, which dropped temps by a good amount. However, the drops still happen. Notably, I noticed a big CPU spike at the moment of the FPS drop, but it quickly returned to normal while the lag continued. All temps seem fine, and I'm worried it might be a deeper issue. I plan to check the BIOS shortly and would rather not replace the CPU if that's the problem. Any thoughts on what could be causing this?