Macbook randomly switches itself off since replacing its screen?

Hi,

The screen of my four year old Macbook cracked the other day and, after being quoted around £500 from Apple to replace it, I decided it was pointless seeing as though the machine cost £700 nearly four years ago.

A friend of mine ordered a screen and replaced it for me. I've just turned it on for the first time an hour ago and the screen and everything else works, but for whatever reason the machine just switches itself off every now and then. Does anyone have any idea why?

I assume it must have something to do with how much of the processor is being used because I have done a little testing and have found that, when the machine is hard at work, it switches off after about ten minutes, but I've been listening to iTunes, browsing the internet, and transferring large files from my web server to my desktop since then and I haven't had a problem. Maybe the problem has stopped? Maybe it can't handle too much processing?

So I'm just wondering if anyone can help me diagnose the problem, and better still, help me resolve the issue. I know taking the Mac to the Apple store would have been the safest solution but surely you can understand my unwillingness to do so considering the machine is worth less than the price to fix it.

Many, many thanks in advance for any help.

Additional info:
The Apple logo at the back of the screen is very dim compared to normal.

Overheating will cause the machine to shut down but that's not the only reason. Bad memory, defective power supply could also cause shutdowns. Replacing the screen may have something to do with it although it's unlikely. Ask your friend to go over the installation to make sure everything is in its right place.

Hmm...

I suppose it's also possible that something went wrong when the screen actually cracked?

I don't know how it happened but parts of the case are cracked slightly.

Just wondering if there's anything I can test out to fix it before taking it to the apple store?

I actually managed to go about six hours without the machine switching off while I was listening to music, uploading/downloading large files to a server, and browsing the internet, so it mustn't be completely knackered.

But just wondering if there's anything I can do if the problem persists (ie. deleting certain files or resetting a setting somewhere etc)?

Thanks for the help.