Severe Frame Drops, Engine Slowdown

Hey! Recently started playing unbound, and while I've seen some stuff about memory leaks, I'm not sure what's causing my issue. I'm running around 90 frames normally, but at intervals every few minutes, the game drops to about 19/20FPS, and the game seems to enter slow motion. Audio and effects are normal, but the game runs slowly. I've monitored system resource usage as this happens, and there's no memory overrun, no high usages in the CPU/GPU, or anything else that would suggest a hardware power problem. This more or less makes the game unplayable, because it jumps from slow motion to normal speed at random points. 

I've tried all I can think of - game settings don't have an effect, I've checked all of my hardware drivers, I've reinstalled the game, ran clean boots, disabled all overlays and anything that would touch the program, and I've had no luck at all. I love this game and I'd like to continue playing it but this is a massive issue.

Has anybody else had this problem? I've seen lots of performance issues, and a few friends have had some, but never anything like this, and since my resource usage is fine it can't really be a memory leak. Any advice? Thanks in advance.