mac powering off problem (-- is it likely to be the hdd?)

[Daniel Johnson]

Hi All

i'm hoping to get some help with a problem I am having with my mac book pro.
Some Specs:
MacBookPro (model identifier seems to be 3,1)
Intel Core 2 Duo, 2.4GHz
2gig memory
SMC Version 1.16f8
160gig HDD - ST9160823AS (split into 2 partitions: Mac partition 123GB, Journaled HFS+, Windows partition FAT32)
OSX 10.5.6

The problem is that my mac, from time to time, just turns off in the middle of whatever I am doing (the screen goes black and the computer is off - this happens instantaneously). Then, when I go to power up, it begins the boot sequence - grey screen, spinning cog/gear - and then turns off again (black screen, no power).
On some occasions, if i try to power up enough times it will boot. On other occasions, many, many power up attempts do not work.
However, when in this state, it will still boot to the windows partition, or boot from an external drive.
Also, when i was booted into mac OS from an external drive, I ran disk repair on the Macintosh HD and got the 'Invalid Sibling Link' error.

After the invalid sibling link error, I ran fsck_hfs - r in single user mode. This found and fixed a number of errors.

Unfortunately, it powered off again. But recovered without me having to resort to fsck_hfs.

Most recently, i ran applejack (auto) in the hope that this will help resolve things.

I also tried resetting the SMC.

So, my main question is -- can I be certain the harddrive is the cause of the power offs? Or is it possible that something else is causing the power-offs and the harddrive errors are a symptom, rather than the cause of the power-offs?
Aside from running the hardware test (holding D with install disc on boot), which I have done (and nothing showed up) is there anything else I can do to try to isolate the problem?

If i can be reasonably sure it is the harddrive I will buy a replacement.

Also, not sure if this is relevant, but the poweroff only seems to occur when the laptop is running on battery.

Any help or advice would be much appreciated.

Thanks
Daniel