OBS local recording making me drop over 100 fps.

first of all, sorry if this isn't the best place to ask, thought it would be though.

I've been trying to record Overwatch at 1080p60 using OBS, using the x264 encoder, at 12000 bitrate, a custom buffer size of 12000, and a keyframe every 1 second. I am not recording to the same HDD Overwatch is on.

when recording, it's fine for the first minute or two, then all of a sudden my fps tanks, and my game and recording stutter, making the game hard to play, and the recording useless in some parts.

I've tried using NVENC, but it's been worse.

I've also dropped settings from EPIC on OW, to HIGH, with the same problem.

My PC specs are:

Intel i7 6850k @ 3.8-3.9 GHz
2 GTX 1080s OC'd
16Gb DDR4 @ 3200 MHz
Any ideas as to why my computer is turning into a potato after a while?

https://pastebin.com/RtRCeRpH (the most recent log file I've got, recordings from today)
https://pastebin.com/cV5UFp3K (logfile from a day or 2 ago, this was when it was working alright, and actually wasn't dropping frames, or at least not over 100)