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After Lion install Macbook shuts down by itself

Time:November 30
At random times My Macbook just shuts down for no rhyme or reason.  Then I have to wait for a few minutes before it will turn back on.
After upgrading to Lion, my computer overheated and I had to have my graphics card replaced.  It's happening again, and I'm using a fan control program to keep my system cool.  This is a bandage and I'll be asking Apple to look into this because there must be something wrong with Lion.
I have done the resets, both the SMC and PRAM/NVRAM.  No effect, and that's true for the original video card and the new one.
I'm tempted to use the Firmware Restoration, but will wait on Apple support.
I have an iMac 24" from ~2006.  It's worked like a charm until Lion.