Rendering massively slows down during final samples

I’m rendering multiple frames to make an animation and the rendering process slows right down on the final 16 samples. it doesnt matter if i use 128, 256 or 512 as soon as it gets to those final 16 it slows to a crawl.

One thing I’ve noticed is even though i’m rendering on the GPU with supported features (GTX 750 Ti) the CPU (AMD A6 6400k) hits 100% load when things slow down according to the task manager.

This doesn’t happen to everything I render most of the time all the samples process at a consistant speed, the only thing my troublesome files have in common is when ive brought objects in from another blend file, weather that was by copy/pasting from one file to another or appeding the object into the current blend file.

Any help or advice would be greatly appreciated as im more concerned about the intense CPU usage than the render speed as my PC has rebooted itself twice whilst trying to render under these conditions.

Using version 3.0, rendering incycles

This is the file im currently trying to render if you want to test this yourself