macbook shutting off randomly

My macbook started shutting off at random times while in use a few weeks ago and I don't know what's causing it. I started having heat issues earlier in the year but installing dmc fan control has seemed to keep it in check and my temperatures have been in the 40s & 50s C since, so I don't think thats the issue. It happens when I only have Safari running, so isn't related to software running. Is there a crash log that can give me a clue? Sorry I'm no administrator so don't know how to figure out whats causing this.

My laptop is a 15" Macbook Pro 3,1 with 2.4 GHz Intel Core 2 Duo, 4 GB 667 MHz DDR2 SDRAM and I'm running OS X 10.9.5. My 500GB HD is only 40% full.

I appreciate any help anyone can offer!

This is a perfect example of why using band-aids like SMC fan control is nearly always a bad idea. Take the machine in for service to replace the non-functioning or clogged fan as you should have done when this problem first started, and cross your fingers you haven't damaged the CPU or logic board.

not a heat issue

What I meant is that I don't think its a heat-related issue. Before installing Fan control my temps were really high but once I installed it my fans are functioning properly and my temps have been low since. Its something else making my computer shut off. It just dies and I have to restart and I don't know whats causing it. How do I figure out whats causing this?

Your description sounds exactly like the emergency shutdown designed to save the processor when it overheats -- due to a malfunctioning or clogged fan, as I suggested previously.

How do you know your temps were "really high"? What do you think is normal?

SMC fan control doesn't fix a malfunctioning fan. It masks problems, and can *temporarily* keep the machine running until a professional can actually fix the issue. As previously suggested, you are at that stage.

Maybe you have a faulty power supply -- from running the fans so much -- or some other problem causing the specific sudden shutdown, but your mentioning of the heat issue and the sudden shutdown sounds like 2 + 2 = 4 to me. I repeat my initial advice, which is to take it in for servicing before you completely destroy it.

before installing smc fan control my macbook was getting up into the 90s (C) and did shut off a few times but since installing smc I haven't seen the temp get above low 60s (I look at it pretty often while using it). I'm planning to buy an iMac soon and don't want to spend the money to have a professional service this machine, but I'd love for it to still be useful to my kids for light schoolwork & internet. Anything I can do? Thank you!