Thank you very much for the suggestion. I had another freezing incident not related to streaming, and I was able to find it in the log. It is described below. Something strange is going on. I have a 1000 GB Dropbox account that mirrors a dropbox folder on an external 2 TB Lacie HD. Inside the dropbox folder I have a much smaller folder, called dropboxmini, which is synced to the internal SDD to gain speed. I use the professional version of the SyncTwoFolders app and use automatic synchronisation. In addition I use Apple's Time Capsule and Time Machine for backup via WiFi.
I did not touch the computer when it froze.
6/4/15 6:32:06.460 PM Mail[233]: Failed to start the SASL connection
SASL(-1): generic failure: XOAUTH2 requires user
6/4/15 6:33:37.000 PM kernel[0]: process WindowServer[155] caught causing excessive wakeups. Observed wakeups rate (per sec): 203; Maximum permitted wakeups rate (per sec): 150; Observation period: 300 seconds; Task lifetime number of wakeups: 185220
6/4/15 6:33:37.214 PM com.apple.xpc.launchd[1]: (com.apple.ReportCrash.Root[3434]) Endpoint has been activated through legacy launch(3) APIs. Please switch to XPC or bootstrap_check_in(): com.apple.ReportCrash.DirectoryService
6/4/15 6:33:37.531 PM spindump[464]: Saved wakeups_resource.diag report for WindowServer version ??? (???) to /Library/Logs/DiagnosticReports/WindowServer_2015-06-04-183337_Franks-Mac-Pro.w akeups_resource.diag
6/4/15 6:36:14.000 PM bootlog[0]: BOOT_TIME 1433410574 0
6/4/15 6:36:15.000 PM syslogd[38]: Configuration Notice:
ASL Module "com.apple.AccountPolicyHelper" claims selected messages.
Those messages may not appear in standard system log files or in the ASL database.
