Hi
After speaking with others who mentioned that the USB HD audio can also cause issues, I removed that cable but it made no difference. It is only the USB 3 cable that causes the frametime spikes.
I did also try an Asus Z590 motherboard and as soon as I plugged the USB 3 cable in; the spikes came back. This at least ruled out the motherboard.
It looks like it is a grounding issue with the front panel, be it just mine or all of them.
The problem is most people who have a case that does this, will be trying every other fix there is and even swapping parts out like I did, not knowing that it's the USB 3, because let's face it, it's the last thing I would ever think of.
The original person who told me about this had a Phanteks case, so it's an issue that can appear on all makes, really, but I am just unsure if it's just my luck all the entire line?