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#11
I'd suspect the PSU actually and test that first since it may be easiest. Just because there is no visible blown caps doesn;t mean that something didn't fry. And just because there is V on all rails doesn;t mean that there is any A to speak of. WHen the machine starts pulling the amps, the PSU may just be shutting down.
On the other hand, a short on the MB will also shut the PSU down so it could be either...
If you have a spare just try replacing that first, then go for the MB (since that is a LOT harder to deal with including reinstalling the OS and everything)