2009 model, that would be about 4 years old.
It sounds like what is occuring is the exhaust vents are clogged with dust and it's overheating the machine causing it to crash.
Your going to have to take the machine to a local PC/Mac repair shop and have them evaluate your sistuation and give you a estimate. Take a look at your RAM and hard drive.
Apple mostly deals with hardware under warranty/AppleCare, then usually hardware, not software issues all that much, then it's a erase and install. Older machines like yours they don't bother or quote a high repair cost and thus expect you to buy a new one.
The local Mac repair shops can give you more tailored attention and perhaps cost effective solutions.
My advice for that machine would be to revert to Snow Leopard, as 10.7+ are a bit of a drain on older hardware, but if you have too many files in 10.8 then your screwed basically.