I tried with another adapter and with another monitor, the problem is still there. Today it happened two times, the first time i was scrolling over an internet page (the main page of this forum, actually), the second time i downloaded a pdf file in my email and i was scrolling over that file. So the scroll operation seems critical. In the log i find something like
12/06/15 19:27:10,359 com.apple.WebKit.Networking[401]: Stream 0x7fe70a4885d0 is sending an event before being opened
12/06/15 19:27:15,991 com.apple.WebKit.Networking[401]: Stream 0x7fe70b028510 is sending an event before being opened
12/06/15 19:27:16,592 com.apple.WebKit.Networking[401]: Stream 0x7fe70b3b5960 is sending an event before being opened
12/06/15 19:28:15,433 com.apple.WebKit.Networking[401]: Stream 0x7fe70b34feb0 is sending an event before being opened
12/06/15 19:28:15,552 com.apple.WebKit.Networking[401]: Stream 0x7fe70a4586f0 is sending an event before being opened
12/06/15 19:28:31,685 com.apple.WebKit.Networking[401]: Stream 0x7fe70b011a20 is sending an event before being opened
12/06/15 19:28:31,689 com.apple.WebKit.Networking[401]: Stream 0x7fe70a61b740 is sending an event before being opened
12/06/15 19:28:39,517 CoreServicesUIAgent[369]: Error -60005 creating authorization
12/06/15 19:28:40,377 com.apple.Preview.TrustedBookmarksService[465]: Failure to de-serialize bookmark data file.
12/06/15 19:32:25,364 spindump[380]: Saved wakeups_resource.diag report for WindowServer version ??? (???) to /Library/Logs/DiagnosticReports/WindowServer_2015-06-12-193225_Mac-Pro-di-Mauro .wakeups_resource.diag
and the 'wakeups report' begins like that
Date/Time: 2015-06-12 19:27:31 +0200
OS Version: 10.10.3 (Build 14D136)
Architecture: x86_64
Report Version: 19
Command: WindowServer
Path: /System/Library/Frameworks/ApplicationServices.framework/Frameworks/CoreGraphic s.framework/Resources/WindowServer
Version: ??? (???)
Parent: launchd [1]
PID: 141
Event: wakeups (microstackshots only)
Wakeups: 155 wakeups per second for 291 seconds
Duration: 291.00s
Steps: 280
Hardware model: MacPro6,1
Active cpus: 8
Fan speed: 789 rpm
Powerstats for: WindowServer [141]
Start time: 2015-06-12 19:28:25 +0200
End time: 2015-06-12 19:32:17 +0200
Parent: launchd
Microstackshots: 70 samples (25%)
Primary state: 35 samples Non-Frontmost App, User mode, Thread QoS Unspecified
User Activity: 0 samples Idle, 70 samples Active
Power Source: 0 samples on Battery, 70 samples on AC
68 start + 1 (libdyld.dylib) [0x7fff912ea5c9]
68 ??? (WindowServer + 3966) [0x103fc9f7e]
68 CGXServer + 839 (CoreGraphics) [0x7fff90ee598b]
35 CGXRunOneServicesPass + 1291 (CoreGraphics) [0x7fff90ee55a1]
35 post_port_data + 237 (CoreGraphics) [0x7fff90ee6baa]
35 eventNotificationHandler + 9 (CoreGraphics) [0x7fff90ef17d4]
35 CGXRunOneEventPass + 38 (CoreGraphics) [0x7fff90eef4da]
34 CGXGetNextEvent + 493 (CoreGraphics) [0x7fff90b9422e]
34 CGXFilterEvent + 81 (CoreGraphics) [0x7fff90c35edd]
34 CGXProcessHIDEvent + 161 (CoreGraphics) [0x7fff90eef295]
34 CGXFilterEvent + 81 (CoreGraphics) [0x7fff90c35edd]
34 sPostContinuation + 7230
I tried almost everything, i changed mouse, changed keyboard. It seems the problem could be labeled as a 'wakeups problem'. I am also thinking the unit is defective but at the apple support center they could not find anything to repair, may be a deeper analysis is needed.