My computer keeps shuting down, please help

My computer will turn it self off because the core temperature keeps raising to high. Its only does it when ever i use high CPU usage stuff like games and burning programs.When im not it will sit at a good 25 degrees. I knows its not a ventilation problem because i have cleaned the case of dust, i have many fans,i replaced the cpu fan with a brand new Blue Orb II, i applied new thermal paste/grease.I dont know what else to do

well i found that the problem was the gfx card, i switched my HD 4870 over with my old 7600GT and everything works fine

open the door if the system runs longer its the psu

check the psu is it blowing very hot air? it may be overheating and your power drops with temps.

the blow orb may have affect the psu cooling or you need more tc or the heat sink is on but not quite right

I had a similar issue and it turned out the the Heat sink I was supplied was not rated for my CPU (AMD 64 939 4000+), it turned out to be rated for AMD's in the 2400+ range.

I upgraded to a Coolermaster Heat Pipe and have never looked back.

I am currently running my system in a Coolermaster Centurion 590, which offers excellent airflow.

I have 4 120mm case fans (1 intake, 1 exhaust and 2 blow holes) with space to add another 2 on the side panel if needed.

I would highly recommend this case or the replacement which is a Centurion 690 IIRC.

Very roomy and lot's of space to hide the cabling and improve airflow etc.