Macbook Pro 13 Inch Shuts Down Randomly??

Ive had my Macbook Pro since June 2009 and about 6 months ago, it started to shut down unexpectedly. It would ask randomly "Are you sure you want to shut down your computer now?" and if i dont click "cancel" right away, it will shut down on its own. When I try to turn the computer back on, it will shut down before it gets to the desktop. The only way I have found that I can get it to log onto the desktop is by continuously pushing random keys while it is booting up. For some strange reason, if i am typing or clicking on my computer while it is on, it will not shut down, however, if i leave it unattended for a few minutes (and sometimes a few seconds) it will shut down. It does this when it is plugged to the charger and when it runs on battery. This problem happens every once in a while. When it first began, it lasted for about two days and stopped for 2 months then again for about a day and stopped for a month and then again for about a week and stopped for 2 more months. Now its started back again and I dont really know what to do. Ive reset the PRAM and rebooted with the Snow Leopard Install CD. Ive run the disk utility and attempted to Verify and Repair the disk and disk permissions. Ive run the programs Onyx and MacKeeper and I cant seem to get this to stop. I took it to the peachmac store and they said that it may be a faulty logic board. But it costs $500 to get it replaced which is 1/3 the price that was originally paid for the Macbook Pro. So please let me know of any thoughts or suggestions to fixing this problem.

Unfortunately, a problem like you're having really needs to be diagnosed by an Apple tech who has the equipment and know how. It does sound like you may have an intermittent defect with the logic board.

When a computer shuts down the way yours is doing, it's usually a hardware problem. It could be bad memory, a defective power supply, it may be overheating and shutting down, or as stated above, the logic board.

Is there an Apple store near by where you can make an appointment and take it on in for them to look at?

Yes there is a store nearby. I guess I'll have to take it by there and see what they say about it. Thank you for answering