By looking for answers over the Internet, I got some tips on Microsoft Answers and I'm pretty much sure my problem was caused by Intel Desktop Utilities. I disabled it and the problem stopped, the latency has not gotten higher than 1200 microseconds and usually doesn't get more than 200 (the peaks are around 600). The version I was running is 3.1.4.031a. Hopefully Intel will improve it on the next versions. I also had another issue with it, that I already posted: one of the temperature sensors had a default treshold of zero degrees C, and even if I changed it, the next boot it reset to zero. So I had to disable the notification for that particular sensor.