I need a CPU intensive task/script

My old Macbook 5,1 (2008) has a problem, it randomly shuts down. For doing the test I installed original Leopard on a formatted hard drive, so its not a software issue. Usually it shuts down when I work on it, but sometimes I find it shut down when I wake up in the morning the second day. Even the new installed Leopard doest not stay on until the morning, despite that I disabled hard drive sleep, disabled screen saver and power saving mode.

A service that repairs electronics suggested that it could be because of CPU overheating and I should service it for Changing Thermal Paste and cleaning the fan that cools the CPU.

Other hardware issues: I replaced the original battery 6 months ago and bought one made in China, but the icon reads "Not Charging" despite that it does charge really slow. I also re-soldered the original magsafe and I bought another one made in china.

So I want to test if the reason why my Macbook shuts down randomly is overheating, and I need a program that consumes lots of CPU continuously.