Hi guys, I think I found a big clue:
I built the pc using an Asus Xonar card first and thinking it was the card I went for the SB Z but the issue remained, so it wasn't the card.
It is very easy for me to reproduce: start a youtube music video and then open the CPU.etl. Stuttering starts as soon as wpa finishes loading the file and whenever I start selecting curves or zooming in.
Weird thing is I have no problems in high end games like Battlefield 4. Only on "lower-end" games (like bejeweled) the problem appears. And I can feel the stuttering in the mouse cursor when it happens (it jumps a bit).
Anyhow, tonight I tried moving the card to other PCI slots and there were no changes... until I tried the PCIx8 slot (usually used for a second video card).
Now I can't reproduce the stutter unless I really stress the system (disk access, CPU, etc.), which should be normal I believe.
Now I'm thinking it's maybe a Motherboard problem? The short PCI slots should be good enough for a sound card?
(I also tested the BIOS settings for the PCI ports moving them from Auto to PCI Gen 3 speed, etc.)
At least it's more stable now, but I wonder if it's still a problem just masked by using a PCIx8 port and I also wonder if it will affect the first PCI performance (video card) because it will go from x16 to x8 speed.
Any suggestion is welcomed.
Thanks!