Heat problem, suddenly shuts down pc

So my friend having a problem on his pc.

the Temperature of CPU was 70% - 80 % CONSISTENT . even we open the vent of casing. and when he play games without a fan steady to his pc the temp is 90%-100% when it reach that it'll sudden shuts down. he don't know what is the cause of shutting down of his pc.

i think there is a problem with HSF?? first the Sink is too thin. and the RPM is 2000+ (the recommended is 5000 not sure) ...

i have read a lots of forum that computer suddenly shuts down.
the causes are the HSF, PSU, and the dirt.

but the PSU is Bnew so there is no problem on that.

and even when cleaned the whole pc. it's still heating up. climbing the temp! .

i'd also check the BIOS to see if the CPU is overclocking but it is not.
i'd reset the BIOS settings. still have the problem

we've changed the Thermal paste.. still got the problem.

we've add some exhaust fan to the PC CASE..still got the problem

HELP us to solve this problem. :sad::sad::sad:

this is the specs:

Mobo : ASUS-PK5PL/1600

4gb Kingston 800Mhz

Pentium D 3.0

PSU 600W True Rated - Forgot the brand

Geforce GTS 450 1gb

USING SPEEDFAN :smile::smile::smile:

Thank you guys!!:rofl::rofl::rofl::rofl::rofl: