Computer completely shuts off while gaming

Okay, so when I'm playing many different games, my computer will literally shut completely off: No blue screen of death, no warning, nothing. Then it will start back up after about 3 seconds of being off. I have checked my CPU and GPU temps while running, and neither seem to go very far above 60 Celsius, so myself and friends don't think it to be a heat issue.
Now for the specs:

Windows 7 Home Premium
Intel Core2 Quad 2.83 (decent CPU fan attached; not stock)
4G RAM (Corsair)
NVidia GeForce 250 GTS
DP43TF Intel motherboard
1TB Hard drive

Also, I saw this in the event logs:
Log Name: System
Source: Microsoft-Windows-Kernel-Power
Date: 4/5/2010 2:50:22 PM
Event ID: 41
Task Category: (63)
Level: Critical
Keywords: (2)
User: SYSTEM
Computer: Me
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="2010-04-05T18:50:22.925613100Z" />
<EventRecordID>17367</EventRecordID>
<Correlation />
<Execution ProcessID="4" ThreadID="8" />
<Channel>System</Channel>
<Computer>Ryu</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>


Any help? We're thinking it has to do with my power supply....

 

What kind of psu do you have in the system? Have you changed any hardware recently? This is a great tool for seeing if you have adequate wattage, but you need to pay attention to amperage also. It could possibly be that, as it sounds like you're thinking, you have enough power to boot and run at normal levels, but once you get that 250GTS humming you lose it.

 

Yup, roomie was barely able to get a look at the Watts labeled on this sucker. 300.....this thing is OLLLLLLD! And @ that tool, yeah I used it before, and did it one more time today. I need around 400-500 to be safe 'n stable. Problem in process of resolving!

 

Agreed, 300 watts is not enough for that machine.

I'd be looking at more 500-600watts to be on the safe side, preferably with 2x 12v rails at 18A+ each.

 

Nedlamar is right on with the higher power; the psu tool is an estimate of min power, and I'd tack an extra 100W on whatever it gave me to be on the safe side- as a minimum. Since you want to overshoot the minimum, I'd go higher.

For perspective, if it helps, my system is similar to yours with a quad core and a GTX260. 600W barely gets it off the ground, I'm currently using an 850W with 2 +12V rails @ 65A. If the small jump in price isn't a problem, if you looked in the 650W-750W range, you'd be giving yourself some wiggle room for upgrades on down the line.