I use a 8800gt, an amd 64 dual core 5000+, and a corsair 450. recently, when I play games such as cod5, red alert3 and gta 4, my computer goes 'beep' and freezes. It always does that when I play games, and I've played these games for more than a month just fine till now. This never happened to me before, I've used the same hardware almost a year and I don't know what the problem is.
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look at TEMP, dowload CoreTemp and monitor GPU temp to.
Monitor your voltages while you play, if you can. Go ahead and look at your temps too. See what you find. May be PSU, may be heat.