Dust in the cooling fans is a factor as mentioned above. Once that is ruled out. Check you RAM slots for dust and clean that as well. Next we need to find out if your RAM has gone defective.
To test that download the ISO and follow instructions from this site http://www.memtest86.com/download.html
If the RAM works fine as well, next step would be your HDD.
Some other test would be.....
1. try using the laptop without the power connected.
2. try with power connected but without the battery plugged in.
I guess some of the Guru's here would do this in a different order. [Please do correct me if it was wrong].