System Stuttering Doing My Head In

Hi everyone,

I am having a frustrating problem with my installation. I did a fresh install about a month and a half ago.

After a few hours of uptime, my PC stutters really severely if I open a random app.

I turned off my router and restored from a previous build, then disabled internet access in that build, and it seems to work fine. However, as soon as I connected to the internet, it starts to happen, so I thought it was probably an AV update issue.

Firefox works fine, for some reason.

I thought I solved it by removing my paid AV (BitDefender), but I was wrong.

Can anyone help? I don't know what to do.