Thanks.
Yosemite Cache Cleaner (Northern Softworks) clears system and/or user cache, when cache is suspected as a fault. This application was run (not installed) for the first time only after I encountered these problems.
Here's the EtreCheck report:
Problem description:
Shortly after login, perhaps exactly when the screen is set to power off (5 minutes), the screen appears black with a moving cursor. Cannot get to a login screen or desktop. The system is set >not< to go to sleep. BitTorrent Sync is shown as a high CPU offender, but has not been running when the screen went dark before.
EtreCheck version: 2.2 (132)
Report generated 6/30/15, 7:47 PM
Download EtreCheck from http://etresoft.com/etrecheck
Click the [Click for support] links for help with non-Apple products.
Click the [Click for details] links for more information about that line.
Hardware Information: ℹ️
iMac (20-inch, Early 2008) (Verified)
iMac - model: iMac8,1
1 2.4 GHz Intel Core 2 Duo CPU: 2-core
4 GB RAM Upgradeable
BANK 0/DIMM0
2 GB DDR2 SDRAM 667 MHz ok
BANK 1/DIMM1
2 GB DDR2 SDRAM 667 MHz ok
Bluetooth: Old - Handoff/Airdrop2 not supported
Wireless: en1: 802.11 a/b/g/n
Video Information: ℹ️
ATI Radeon HD 2400 - VRAM: 128 MB
iMac 1680 x 1050
System Software: ℹ️
OS X 10.10.4 (14E46) - Time since boot: 0:7:51
Disk Information: ℹ️
ST3750640AS disk0 : (750.16 GB)
EFI (disk0s1) <not mounted> : 210 MB
Recovery HD (disk0s3) <not mounted> [Recovery]: 784 MB
Backups (disk0s4) /Volumes/Backups : 499.87 GB (386.62 GB free)
Macintosh HD (disk1) / : 248.83 GB (226.59 GB free)
Encrypted AES-XTS Unlocked
Core Storage: disk0s2 249.16 GB Online
USB Information: ℹ️
Western Digital My Passport 0748 2 TB
EFI (disk2s1) <not mounted> : 210 MB
Paul (disk2s2) /Volumes/Paul : 2.00 TB (630.69 GB free)
American Power Conversion Back-UPS BR1500G FW:865.L2 .D USB FW:L2
Apple Inc. Built-in iSight
Apple Inc. BRCM2046 Hub
Apple Inc. Bluetooth USB Host Controller
Apple Computer, Inc. IR Receiver
Firewire Information: ℹ️
PI-367 USB2.0 & 1394 & eSATA combo Drive 800mbit - 800mbit max
EFI (disk3s1) <not mounted> : 210 MB
Boot OS X (disk3s3) <not mounted> : 134 MB
iTunes (disk4) /Volumes/iTunes : 3.00 TB (392.01 GB free)
Encrypted AES-XTS Unlocked
Core Storage: disk3s2 3.00 TB Online
Gatekeeper: ℹ️
Mac App Store and identified developers
Kernel Extensions: ℹ️
/Library/Extensions
[loaded] com.sophos.kext.sav (9.2.50 - SDK 10.8) [Click for support]
[loaded] com.sophos.nke.swi (9.2.50 - SDK 10.8) [Click for support]
Launch Agents: ℹ️
[running] com.sophos.ui[redacted].plist [Click for support]
Launch Daemons: ℹ️
[running] com.bombich.ccchelper.plist [Click for support]
[running] com.crashplan.engine.plist [Click for support]
[running] com.netputing.handyPrintSystemDaemon.plist [Click for support]
[running] com.sophos.common.servicemanager.plist [Click for support]
User Login Items: ℹ️
iTunesHelper Application (/Applications/iTunes.app/Contents/MacOS/iTunesHelper.app)
ClamXavSentry UNKNOWN (missing value)
iTunes Application (/Applications/iTunes.app)
SMARTReporter Application (/Applications/SMARTReporter.app)
HandyPrintProStatus Application (/Applications/handyPrintPro.app/Contents/Library/LoginItems/HandyPrintProStatu s.app)
BitTorrent Sync Application (/Applications/BitTorrent Sync.app)
CrashPlan menu bar Application (/Applications/CrashPlan.app/Contents/Helpers/CrashPlan menu bar.app)
Carbon Copy Cloner Application (/Applications/Carbon Copy Cloner.app)
HazelHelper Application (/Library/PreferencePanes/Hazel.prefPane/Contents/MacOS/HazelHelper.app)
CCC User Agent Application (/Applications/Carbon Copy Cloner.app/Contents/Library/LoginItems/CCC User Agent.app)
Internet Plug-ins: ℹ️
Default Browser: Version: 600 - SDK 10.10
QuickTime Plugin: Version: 7.7.3
3rd Party Preference Panes: ℹ️
Hazel [Click for support]
Time Machine: ℹ️
Skip System Files: NO
Mobile backups: OFF
Auto backup: YES
Volumes being backed up:
Macintosh HD: Disk size: 248.83 GB Disk used: 22.23 GB
Destinations:
Backup [Network]
Total size: 997.71 GB
Total number of backups: 53
Oldest backup: 2015-04-20 02:44:36 +0000
Last backup: 2015-06-27 04:47:07 +0000
Size of backup disk: Excellent
Backup size 997.71 GB > (Disk size 248.83 GB X 3)
Top Processes by CPU: ℹ️
93% BitTorrent Sync
10% Activity Monitor
10% ScreensharingAgent
10% WindowServer
7% iTunes
Top Processes by Memory: ℹ️
334 MB kernel_task
242 MB BitTorrent Sync
233 MB iTunes
184 MB com.apple.WebKit.WebContent(3)
180 MB SophosScanD
Virtual Memory Information: ℹ️
45 MB Free RAM
3.95 GB Used RAM
0 B Swap Used
Diagnostics Information: ℹ️
Jun 30, 2015, 07:37:38 PM Self test - passed
Standard users cannot read /Library/Logs/DiagnosticReports.
Run as an administrator account to see more information.