PC slow when multitasking/gaming, with distorted sound

My PC has Windows 7, 8gb ram, an intel i5 4670 and Nvidia GeForce GTX 660 graphics card, corsair 650w PSU, and 256mb solid state drive as well as some other western digital hard drives. It's around 3 years old at this point.

Recently I swapped my old 19" 4:3 monitor (I know, I know, but I liked that monitor) for a bigger screen with a 1920x1080 resolution. Since then it's been suffering from intermittent slowness and distorted sound.. For example, when playing a game (Archeage, an MMO, 2 year old game so not hugely cutting edge) and trying to play music at the same time from youtube, with several browser tabs open, the game's FPS drops and the sound will become distorted and slurred. I turned down the game's graphics settings from 'very high' to 'normal' which fixed it for a while, but overall it's progressively gotten harder and harder to avoid my PC sounding slurred and distorted and going slow during gaming. I used to be able to reasonably smoothly run two instances of this game at once to multibox, and now I can't even reliably run one without this problem if I do anything else but play the game, and it keeps getting worse.

At first I thought it was overheating, because GPU and processor both typically were maxing in the low 80s (Celsius). I cleaned out a lot of dust from my CPU and GPU fans but now I'm finding that I still get distortion and slowness even though the temperatures are 10+C better. I even got distortion when both were at 50C.

I tried benchmark my PC using novabench and for what it's worth it scored as entirely normal using that software.

I ran malwarebytes, which found I had a potential malware/adware known as 'mobogenie', but now that it's been removed I still have the same issues.