Freezing

I'd like to consider myself fairly knowledgable about computer equipment but I've hit a wall here. I've had issues with my computer freezing and requiring restart. I'm suspecting PSU, but I'd like to ask your opinions first. This issue has occurred for over 2 years now intermittently depending on the load on the system. The motherboard has been replaced, different ram has been used, the cpu and graphics card have been upgraded. The only constants are the raid hard drives and the power supply. The system specs are as follows:

*BFG 680i motherboard
*Q9550 overclocked to 3ghz
*6GB Ram (generic/ocz/corsair - it doesn't matter what I put in it) -run between 1.8-2.1v
*Ati HD 4870
*2x 74gb Raptors
*1 500gb seagate sata drive
*Antec True Power II 550EPS powersupply (19a on each 12v rail)
*1 DVD-RW
*Has occurred on XP/Vista/7

So the problems tend to occur when there are more things happening on the computer. Specifically if I'm playing a game, like world of warcraft, and running media center on another screen or watching a flash video at the same time the system just hangs. The event viewer log states the following:

Log Name: System
Source: Microsoft-Windows-Kernel-Power
Date: 11/6/2009 11:32:31 PM
Event ID: 41
Task Category: (63)
Level: Critical
Keywords: (2)
User: SYSTEM
Computer: Amin-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>2</Version>
<Level>1</Level>
<Task>63</Task>
<Opcode>0</Opcode>
<Keywords>0x8000000000000002</Keywords>
<TimeCreated SystemTime="2009-11-07T07:32:31.259200000Z" />
<EventRecordID>28938</EventRecordID>
<Correlation />
<Execution ProcessID="4" ThreadID="8" />
<Channel>System</Channel>
<Computer>Amin-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">false</Data>
<Data Name="PowerButtonTimestamp">0</Data>
</EventData>
</Event>

Flash video and media center running simultaneously seem to be the culprit most of the time. However the problem has occurred at other junctures when these two are not present. I've been a tad out of the community for a while and I'd really appreciate any help. Thanks .

I failed to mention that the problem occurs at stock settings as well.