OK, got the new 4x4 Dominator GT 2133 kit installed. unfortunately the sleep and shutdown issues seem to persist. This is both happening at my OC (4.7 on CPU, 2133 on memory) as well as at both stock CPU and memory speeds.
Not really sure where to go from here... I will say the system is rock solid stable, I can stress test all day without any crashes or instability at both stock speeds and at the overclock -- the only issue is that the system doesn't power down on shutdown properly sometimes, and also either doesn't power down into sleep state and/or restore from sleep state intermittently.
Something really strange happened a bit earlier today also. I brought the computer into a sleep state, it powered down and the power LED started flashing like it successfully entered sleep. I walked away for a minute or two, came back, and the computer power was running again - although the system was inaccessible like it tried to wake from sleep unsuccessfully. Checked the Windows event logs again and not seeing anything of interest.
Not really sure where to go from here...any additional thoughts?