Overheating - need help finding source

I want to start with the fact that I'm not ignorant when it comes to computers. I bought and assembled my pc part by part.

A few days ago my pc started shutting down after playing for a while. Not rebooting. Shutting down. I have come to the conclusion that it's definitely not a software problem.

It is curious to note that my hardware hasn't changed since the day I bought my pc, a few years back. Also, the games that make the pc shut down never caused this before. With software and hardware that hasn't changed in months, suddenly my pc is overheating. Curious.

Perhaps too much dust? Maybe my hardware getting less efficient with age? Who knows. I only care about finding the source and fixing this. I can't even play Minecraft for and hour anymore!

I installed SpeedFan and a label that-I-can't-do-anything-but-assume-it's-the-processor got around 94 C while playing. It was here that I realized I don't know anything about running temperatures. Don't know if 94 C is normal. Other than that the program wasn't very useful. Too many unnamed labels (and apparently 2 of my cores are below 0 C).

I suspect the problem is either the processor, the chipset, or the PSU.

My pc:
ASUS P5N-E SLI
Intel Core 2 Quad (2.4 GHz)
2 GB DDR2 RAM
2x Geforce 9600 GT
3x assorted HDD's
Cooler Master PSU (can't remember right now but about 550w or more)

Never been overclocked. Also it has no warranty anymore. So I need good advice. How can I find out the source of the overheating?

As a side note, my PSU's internal cooler started making noises a few months ago. I've read this is due to dust and stuff. Perhaps it's relevant?
 
Remove the CPU cooler-heatsink. Clean off all the dust and any other debris. Clean off the old thermal paste and apply a small amount of new paste. Then check the PC's operation
 
Remove the CPU cooler-heatsink. Clean off all the dust and any other debris. Clean off the old thermal paste and apply a small amount of new paste. Then check the PC's operation

I have no thermal paste. :V

Tomorrow I'll buy some and do that. I'll post my results then.
 
"I bought and assembled my PC part by part"... You must have used the "pad" thermal paste already applied to the CPU stock heatsink, right?

Another good reason to apply fresh thermal paste
 
I should point out I ran SpeedFan again.

There seem to be 5 "Core" labels (I have a Core 2 Quad 2.4 GHz). They go like this:

Core - 57 C
Core 0 - 72 C
Core 1 - 69 C
Core 2 - 71 C
Core 3 - 68 C

They all have the fire icon.

Also:

'Temp1' at 67 C (fire icon too).
'Temp1' (again) at 40 C (green checkmark)
'Temp2' at 35 C (blue down arrow)
'Temp3' at 25 C (blue down arrow)
 
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