Why does my MacBook Pro shutting down by itself?

I've been through the forum and I feel a little stupid asking this, but here's the problem.

I've had this baby for about a year now, and it had started showing signs early on in. The first time she shut herself off, I took her to the guy who sold it to me and he recommended using SMC FanControl, which worked just fine for me for about 3 months. After that, any temperature above 58degrees Celsius and I'd start to get worried.

Fast forward and 4-5 months later, even a temperature of 47 started getting her into shutdown tantrums. Tired of this, I called the guy again and (post reading the forum here) asked him if it could be a problem with the heat sink. To which he very confidently told me that a MacBook Pro A1226 series doesn't HAVE a heat sink! ( 0_0 )

A little scared, I went to another laptop repairman who said that the NVidia display chipset might have been the cause for all this and he proceeded to replace the chipsets (or so he says. I haven't been able to gather the courage to check).

Now when I brought her back home and booted her up, I set up a transfer from my mac to an external hard drive, hoping it wouldn't be too much of a load on her. But she gave up in exactly 23 minutes and shut down again. This time, i hadn't used smc fan control AND had taken the battery out, hoping that removal of the battery would lessen the heat. But as usual, 57 was the maximum she could take. Here, the fans were running at full speed on smc fancontrol(6000 rpm)

So..... bringin my problem to a point,

H E L P!!!

:-(