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My computer randomly shutdown while playing a game.

It started beeping. 3 long beeps, 1 per second, then repeated after 5 seconds.

I just installed a new GFX card (750 Ti), and reseated the CPU heat sink.

How do I diagnose the problem?

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3 long beeps is normally a memory issue but you don't mention what BIOS you have.

Since you've installed hardware, ensure the RAM is seated well.

Try with just 1 stick of RAM, and then swap it to see when/if the beep continues/stops. Then maybe use MEMTEST...

Read more about beep codes

However, I would first ensure everything is seated. It could be the new GPU isn't seated correctly...

Based upon now knowing your BIOS in the comments, here is the same question on a forum which also suggests it's faulty RAM

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I will run MEMTEST. How do I make sure there is enough power? The installed GFX card was a GeForce 750 Ti (upgraded from a GeForce 250), which did require a power input. – Petah May 13 '14 at 10:13
The power requirements are different for every GPU look your up then compare to your PSU – Ramhound May 13 '14 at 10:14
My PC boots up fine, its only under load it crashes, so will changing out RAM effect this? – Petah May 13 '14 at 10:19
@Petah - You need to do multiple passes. It should not fail a single pass. Good memory will contain zero errors ( this is because the error correction that exists). So if you have detectable error it means you have more errors then the error correction can resolve. – Ramhound May 13 '14 at 10:46
What don't you understand? The RAM (a stick or multiple sticks) are faulty. Bin it. Buy new. – Dave May 13 '14 at 11:15

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