Computer crashing, possible PSU issue

Recently, my desktop (which I built myself) has been crashing. This hasn't happened while I've been in the room, so I'm not sure exactly what occurs before the crash.

Anyway, whenever the computer crashes, the computer won't turn back on unless I unplug the cables and plug them all back in (I haven't tested it with individual cables, as the computer crashes relatively infrequently and seemingly only after it's been on and idle for a significant period of time). It'll power on after I do this, but each time I've powered it on after plugging the cables back in the monitor only shows a black screen, though when I restart it after that it runs just fine. I'm thinking it may be an issue with my power supply, or possibly my mobo.

Also, whenever I turn on my computer now, the DRAM_LED turns on briefly. My computer runs normally when I get to work again after it crashes, and it I shut down the computer myself, I have no issue turning it back on.

Here are my specs:
Windows 7 Ultimate
Asus M4A88TD-M USB3 AMD 800G Socket AM3 MB
AMD Phenom II X4 955 Black Edition
Patriot Gaming i5 8GB DDR3 1333 MHz
Ultra LS600 600W ATX Power Supply
Seagate 2TB LP Serial ATA HD 5900/64MB/SATA-6G
NZXT Lexa S Crafted Series Mid Tower PC Case
EVGA GTX 460 Video Card

If any additional info is needed, let me know.

 

This certainly could be a power issue and when it comes to troubleshooting, I always want to verify I am providing good power. So I recommend you swap in a good PSU, or have that one tested.

The black screen could indicated graphics problems - but again, that could be cause by a power problem.

Is the case interior clean of heat trapping dust? What are your temps?

If overclocking, stop! At least until this is resolved.

I am not sure that indicates a problem - unless you are suggesting the LED should stay on.