Resolution changes for no reason (Garry's mod)

I’ve been having a problem for a few months where my resolution changes for no reason.
My standard resolution is at 1920 x 1080 (native) and it changes to 1280 x 720 and then my game continues to have lag spikes, roughly every minute if not more.
I tried reinstalling Garry’s mod, deleted all my addons and even deleted my whole Garry’s mod folder.

Some information about my PC, OS: Windows 10, GPU: GTX 960, CPU: i7 4790k. I hope that is enough information, if more is needed let me know.