Stuttering again and again...

[bluetooth]

yup i agree. i have some trojan which can't be cleared by kaspervy but can be cleared by AVG malware (AVG anti virus can't detect it too).

[M-1]

I'm quite sure it's not my case... I assemble something like 100 PCs in one year and I install the OS always in the same way... I saw this problem ONLY on some of the Gigabyte P965/P35 DS3/R series motherboards. So I belive it's something related to them (or these Intel chipset in some way).
When I fight with a malware/virus I usually can see very strange problems and the PC becomes unrealiable/unstable in a lot of different ways. Here, the PCs are rock stable with no problems at all... except this rare one... for rare I intend that may be one time in one year.
Also, the time when (in front of the stutter problem) I unplugged the ethernet cable and the system freezed until I replugged it lead me to think that there is some kind of very low level problem related to some BIOS routine rather than a malicius software at ring 0.

Anyway, I haven't thanked you for your ansers yet! ;)

[bluetooth]

btw, what sort of power supply do you use on all the boxes . Are they the same brand/specs? what brand/specs?

[M-1]

Let me think... If I'm not wrong they are all from Enhance (360W and 460W single 120mm fan models)... they are generally very good PSUs.