pc wont start

Try replacing the heatsink of the computer which sits on the motherboard. It can be either heatsink or the processor but its a good step to try heatsink first as it isnt as costly.

Yeah, I know what it could be... definitely NOT the power supply. :-)

Are you using a device built for the country you're in?

I think your MB may be toast.  Usually, this is a result of bad power supply (which you seem to have already tested), bad motherboard, or bad processer.  I would venture to say it is the motherboard.  

One thing to try. Do you know how to short the switch on the motherboard?  There will be a connector that goes from the button on the computer case to the motherboard.  If you short those pins with a screwdriver or something, it might come on.  If so, your button is bad.  Otherwise, MB.  SOrry...

Not sure if it was built in the country, Australia i doubt it Why?  
If it is the heatsink does this relate to hot or clod starts?

With heatsink it is not necessary for the PC to go real hot. As the heatsink allows the processor to cool down, it will only do that if it is working properly.

If it is a cooling issue, it should come on when the computer cools off.  ANd it should at least try to start.  I think the problem is more sinister.  Try what I was saying with the screwdriver.  Find the pins for powering the computer on the motherboard.  See if that works.  If not, replace the motherboard.

I agree it may be the motherboard.  

I just thought of another thing.  Sometime I jump the gun with assumptions, but I assume you didn't tamper with anything on the motherboard.  

Some motherboards will do what you are saying if a) the motherboard bios settings do not work or b) the bios jumper is missing.  This isn't always easy to find, but you should take a look at the bios jumber.  If it is in place, follow the motherboard instructions to reset the bios.  

Try removing all memory modules, video cards, etc. I've had unseated memory modules prevent the PC from booting.

I'm not saying not to try it, but I have never had unseated memory or loose add on card cause the computer not to power on.  This is a power issue.  The only other strange thing I can think is literally a loose screw or something shorting something on the board.

Good luck.

I removed cpu, cleaned and so far so good.  Although time will tell in the past it does go for some time then same thing.  Appreciate all the suggests.  thanks