Hmm, I'd never heard of the event viewer. I just opened it to find several critical instances, one in the last day. The same critical event has occurred 74 times since March 14th; these are its details:
Log Name: System
Source: Microsoft-Windows-Kernel-Power
Date: 2/5/2014 5:22:20 PM
Event ID: 41
Task Category: (63)
Level: Critical
Keywords: (2)
User: SYSTEM
Computer: Gaming-PC
Description:
The system has rebooted without cleanly shutting down first. This error could be caused if the system stopped responding, crashed, or lost power unexpectedly.
Event Xml:
<Event xmlns="
http://schemas.microsoft.com/win/2004/08/events/event">
<System>
<Provider Name="Microsoft-Windows-Kernel-Power" Guid="{331C3B3A-2005-44C2-AC5E-77220C37D6B4}" />
<EventID>41</EventID>
<Version>3</Version>
<Level>1</Level>
<Task>63</Task>
<Opcode>0</Opcode>
<Keywords>0x8000000000000002</Keywords>
<TimeCreated SystemTime="2014-05-02T16:22:20.520435400Z" />
<EventRecordID>14714</EventRecordID>
<Correlation />
<Execution ProcessID="4" ThreadID="8" />
<Channel>System</Channel>
<Computer>Gaming-PC</Computer>
<Security UserID="S-1-5-18" />
</System>
<EventData>
<Data Name="BugcheckCode">0</Data>
<Data Name="BugcheckParameter1">0x0</Data>
<Data Name="BugcheckParameter2">0x0</Data>
<Data Name="BugcheckParameter3">0x0</Data>
<Data Name="BugcheckParameter4">0x0</Data>
<Data Name="SleepInProgress">6</Data>
<Data Name="PowerButtonTimestamp">0</Data>
<Data Name="BootAppStatus">3221225684</Data>
</EventData>
</Event>
None of that makes any sense to me but it certainly sounds like it's related to the problem. I haven't noticed any unusual q-code readout on my motherboard as the system hangs, but I'll reboot and check it again now.
There are also lots of entries under the 'Error' category in the Event Viewer; 80 instances in the last 7 days. I don't know if this is normal but it doesn't sound it!
EDIT: I reset the system again and kept an eye on the q-code, but it seemed normal. As it was hanging, it remained in the 'AO' (normal) q-code state. I also timed it: from the moment the display turned off to the moment the motherboard began responding again, 2 minutes and 5 seconds passed (which feels like a lot longer when you're waiting for it!)
EDIT2: I googled the problem and found advice on this link:
http://support.microsoft.com/kb/2028504
The advice was to uncheck 'Automatic Restart' in advanced system settings, which I did. It didn't solve the problem, as the computer still hangs for roughly the same length of time.