mac pro late 2013 freezes with spinning wheel

i bought a mac pro (late 2013) 50 days ago, os is yosemite 10.10.3. I often get the spinning wheel of death and the mac freezes, i.e. it doesn't react to mouse click or keyboard. it happens on average 1 time a day. When it happens i have to restart the machine by pressing the computer's power button.

I have already contacted apple support by phone, i have made many reset operations under their control, but nothing changed.

After the restart, when i examine the console log the last message before the reboot is often the following one

windowserver: surface testing disallowed updates for 10 sequential attempts

By searching this message on google i found that the problem might be related with the energy saving settings, i.e. it happens when the mac wakes from sleep. So i changed those settings to prevent the mac going on stop state, and today the problem did not happen, but i'm not sure what wil happen tomorrow ... so i'm looking for information/advice about this kind of problem.