If you don't feel like reading this long-winded post skip to the bold question at the end. 

 
About 5 months ago I got a new video card for my HP PC and had to get a new power supply as most people do when they upgrade video cards. The new card (ATI Radeon 4500) required a 450W power supply, but I decided to get a 500W just to be safe (Thermaltake purepower is the brand).  About a month after that my 2.5 year old HP computer died - it was the motherboard - and I decided to start from scratch and buy a new one.  I got an Asus Essentio and of course I put my newish video card and power supply in it. 
 
Since then I've had 3 or 4 cases in which the computer simply won't start. I push the power button, and absolutely nothing happens, not even a faint noise of the fan starting.  I usually open up the case, check my wires and cables to make sure everything is hooked up properly, try to follow the wire for the power button, and basically tool around trying to figure out what the problem could be.   After a bit of that, I give up and decide to see if it works again - and wa la! It usually starts right back up. I have not idea what it is, but at that point I'm just thankful it's working and go on and forget about it until the next time. One time I even just switched power chords (from the back of the power supply to the outlet) and that fixed it. I know that both of these power chords work and I've had the problem happen with each of them.   
Last night the computer shut down randomly when I was outside for a few, and wouldn't turn back on. I had finally had enough and decided to get out my multi-meter and test the power supply with that. I followed the instructions on a couple websites 
http://articles.techrepublic.com.com/5100-10878_11-1056686.html  
  http://www.youtube.com/watch?v=rivoVzxwNtI 
Nothing happened - no power!  I messed with some things, switched power chords (back to the one that had failed me last time this happened - 2 weeks ago) and then tried again. This time I saw the fan start spinning in the p.s. and I tested with the multi-meter and it worked. I came to the conclusion that my power supply works sometimes and other times doesn't. Is this possible? Should I contact thermaltake about getting it replaced? 
 
Can a power supply work sometimes and not work at other times. In my other experiences with bad power suppplies, it seems that when they go they go and they aren't coming back. This case is different. Is that possible?

I've had flaky power supplies that get worse with time. It's probably a bad capacitor decaying faster than usual. I don't suspect capacitors affected by capacitor plague appear that often nowadays - more likely it's just a dud.

I've had experiences with 'flaky' power supplies.  Not personally, but with people I've helped out.  I think your problem could very well be with the power supply.  At the very least, it would be worth getting a new one and trying it out as they're fairly cheap.

They're pretty quick to swap out, so perhaps you could borrow one from a friend to check.

I'm also using a cosair 750 watt psu, if I shut down the pc from the back switch and turn it back on, it might take two tries to turn it on however mostly one try. I noticed you need a circuit or current running through the whole pc before it will even turn on. I remember one of my hardrive wires doesn't exactly work, I had to switch it out for another one then it will complete the circuit. I think one of your wires is not working well, that's why it turns off mid use because it's faulty. Also if it doesn't always turn on, the circuit isn't fully going through. You can try to idenfity the wire, that's difficult unless you have multiple connectors like I do, or simply just get a new psu for cheap. It can't be any worst than now, it's not suppose to do that.  
 
Of course if you do get a psu, get a brand name reliable psu, my other psu's fan doesn't even spin anymore. A lousy psu can fry your mobo and cost you even more. I'm not sure i'm 100% correct, however that's my advice.

" 
If you don't feel like reading this long-winded post skip to the bold question at the end. 

 
About 5 months ago I got a new video card for my HP PC and had to get a new power supply as most people do when they upgrade video cards. The new card (ATI Radeon 4500) required a 450W power supply, but I decided to get a 500W just to be safe (Thermaltake purepower is the brand).  About a month after that my 2.5 year old HP computer died - it was the motherboard - and I decided to start from scratch and buy a new one.  I got an Asus Essentio and of course I put my newish video card and power supply in it. 
 

Last night the computer shut down randomly when I was outside for a few, and wouldn't turn back on. I had finally had enough and decided to get out my multi-meter and test the power supply with that. I followed the instructions on a couple websites 

Can a power supply work sometimes and not work at other times. In my other experiences with bad power suppplies, it seems that when they go they go and they aren't coming back. This case is different. Is that possible?

  "