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Random Shut Downs on my PC

I am having problems with the power supply on my PC and would like some advice before I go ahead with what I assume is probably my best solution, ie  buy a new power unit.

The Problem:  random power outs!  Without any warning, at unpredictable moments, the computer will shut down. No messages or blue screens.  Just a clean power out.  It automatically reboots and then powers off again after a minute or two, continuing this cycle with ever shorter periods until it can't get further than the Windows Safe Mode menu.  

The Background: I bought a new system about 7 months ago [Lenovo ThinkStation S10 6423, Intel Core 2 Duo E840, 3 GHz], fitted a new graphics Card (Nvidia Geforce 9600+ IGB HDMI), increased the Ram to 4 gbs, added a SoundBlaster X-Fi Elite Pro sound card, and a Hauppage TV card.  I also swapped the PSU for the one on my previous system (Nesteq Model NA 5201, max 498w) because it had better connections than the one supplied with the ThinkStation, and had "quiet technology" built into it. The ThinkStation came with Vista preloaded, but I went back to my XP 32-bit which I knew and loved, until a few weeks ago when I upgraded to Windows 7.  

Everything was fine until two days ago, when the computer refused to power up.  After a few tries I came to the conclusion that the old Nesteq PSU had finally given up the ghost, so I replaced it with the original PSU supplied with the ThinkStation (650w, but very basic in comparison to the Nesteq).  That's when the trouble started!

Even though the restored PSU has more wattage than the old Nesteq, is it possible that it is still not powerful enough to drive the Graphics/ Sound/ TV cards that I have installed?.  Could there be other explanations for the power outs (surely it has nothing to do with upgrading to Windows 7)?  Or am I simply going to have to fork out for a new PSU?   And if so, what specs do I need to bear in mind.  
Movie Stars

Solution: Random Shut Downs on my PC

Thanks. I will install CoreTemp and monitor temps properly - but when the computer blanked out just now,  I checked and the PSU was really hot.  I'm sure the fan either isn't working properly or (more likely) isn't up to the demands of my system.  (it doesn't even have an on/off switch!)

Fortunately I never throw anything away and remembered that I had an X-Power 600watt  PSU (with Ultra Quiet technology) left over from a previous attempt to upgrade  (I was going to sell it on e-bay - glad I didn't now!).  It's very similar to the Nesteq, and I have installed it and so far so good.  

Fingers crossed!!