Sounds to me like you have a power supply about to head south. It could be any number of other things, like a bad mobo, RAM, or something, but I doubt it.
What I suspect is happening is that you have that rig really loaded with hardware and the power supply, whenever the room temp around it is high or you put a lot of load on it, is going below-spec in output wattage. It gives you spotty failures and the equivalent of brown-outs.
If this is the case, it's easy enough to replace, as you know.
Well, there are power supplies and then there are POWER supplies. Not to mention that a year is plenty of time to have one crap out. Go to http://mbm.livewiredev.com/ and pick up a copy of Motherboard Monitor 5. It's a little difficult to set up, with a LOT if options and not much help on WHY, just HOW. Still, if you right-click on the tray icon after you install it, you can choose to open the Deashboard. It will show you all your voltage levels and system temperatures, too.
Yes, please do! As I said, it could be RAM, and it sure looks like it is. It must be a section that is very close to where the boot code loads to affect the system like that. I got your case mixed up with a similar one where the guy had already run memtest86. Sorry.