My Pro Will Randomly Shutdown

I own a late 07' early 08' MacBook Pro that has iOS 10.6.8(snow leopard i believe) and recently it will just randomly shut down while im in the middle of surfing the web or whatever I'm doing. Its not consistent when it does it but it is reoccurring. Other than that the machine is flawless and has been great in the 5-6 years Ive owned it I've only replace the logic board, everything else has been good. Any feed back is welcomed!

There are two possible issues causing this (in my experience)...

1. a bad battery can cause random shutdowns (even if it's on power)....

2. Sounds to me as if it's got a crack in the main board.. i've seen this on several of the 07/08 Pro 15" units- - random shut offs... There are a couple shops who fix that issue, or you might be looking at a new board....

so i'd confirm the battery (got a buddy with a similar system you can swap batteries with for a couple days?), and if that doesn't do it, get a repair shop.

Another possibility.
Have an '07 MacBook here - had same problem just 3-4 months ago - just randomly shut down.
After loading up SMC fancontrol, found the fan was not doing it's job - tried just cleaning the fan, it was pretty dirty, didn't really do anything - new fan $15 - issue gone.