Yosemite 10.10.5 audio dropouts

Hello everybody!

I just upgraded from 10.7.3 to 10.10.5 on my iMac (half 2011, SSD 256, 16GB RAM, i7 3,4Ghz).

Since I upgraded I hear audio dropouts when listening to music. They happen not so often (one every hour, more or less) and they last just a second... I tried iTunes, Vlc and Cog and it happens with them all. To listen to music I use both a MOTU 828 Mk3 firewire and the built in speakers and it seems it doesnt matter. Sometimes, the audio... drops out.

I already tried the reset of PRAM and SMC, I launched the "killall coreaudiod" process... nothing changes. Sincerely don't know what else I could try. It's annoying.

Any ideas? Thank you.