System stuttering when alt tabbed out of games run in fullscreen after getting new CPU

Oct 10, 2018
I fried my old mobo + CPU so I bought a new pair. I went from an i5-4690k to a Ryzen 7 2700 (non-K). When I tab out of a game which I run in FULLSCREEN, my audio and mouse stutters every now and then. Its unbearable on discord and when trying to browse. Basically my whole system lags when tabbed out of a game in fullscreen. Does this have anything to do with the CPU? It doesnt reach 100% ever, mainly due to me (purposefully) picking a CPU of that caliber to run with a GTX 970.

I have 3 theories:

1) The GPU is maxed out and slows down the system. It runs at 100%. This is unlikely though, as audio stutters as well, and I know that is CPU related

2) RAM is maxed out. Got 16 GB ram, system says only 7.93GB are usable. I am calling the guy who assembled my PC tomorrow regarding this. RAM never exceeds 6.8 GB used no matter how many tabs I open up though...

3) The CPU is not suitable for processing a game in the background and doing other tasks? Thinking this is unlikely as well as it's a ducking 8 core processor