I agree, it's overheating, probably your video card going bad. Check and clean all your fans...I'm going through this right now and the same thing is happening. Think my fan is going bad, so I have another fan blowing on my card and it runs fine, unhook it, only takes a few minutes for it to go black. The "no signal" is the give away and I'm sure it boots up fine until you place a load on it.
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