Sleepy computer

I'm experiencing an odd problem. After pressing the sleep button on the keyboard, the computer will not reawaken. The fans and lights remain lit. I have to turn off the power at the pwr supply, then back on, then sometimes it will start properly. Other times, I have to press a button on the motherboard to get it back running. When it goes to sleep on its own, sometimes it will come back from jiggling the mouse, sometimes not.

I had this problem for a while, but hadn't isolated it, thought it might have been an OS issue, so I did a fresh install. I've made sure that all the drivers and bios are up to date, and that the bios power management settings are correct, as well as the mouse and keyboard settings in device mgr.

Currently, I only have two hard drives installed, down from five, to keep things simple.

I built this a couple years ago.
Components:

Intel i7, 920, Asus P6T
12 gb OCS memory
ATI 5770 GPU,
700 watt pwr supply

Win 7 Ultimate 64bit

I suspect the power supply? I can't locate any connectors that are loose or plugged in wrong.....