Hello. Just registered to say I am having a very similar problem with my OE (Virgin Mobile). I just bought it a few days ago, and it's been working like a charm--until today:
Went home for lunch, put my phone on the table. ~30 minutes later I picked it up and it was completely non-responsive. Black screen. No response to power button. No response to holding power button down for 10 seconds. I pulled the battery and it re-started just fine.
Maybe an hour or so later it happened again. So, I deleted nearly all of the apps I'd downloaded. I had already shut down wifi, bluetooth, GPS, and sync with a power widget, trying to conserve some battery, so I figured it was a misbehaving app. After deleting all those apps, I checked what was running, and I didn't see anything odd. Until...
Less than an hour later it happened again. No idea what's triggering it, but it sounds very similar to the thread starter's problem. I'm thinking I'll head out to Best Buy soon and try another OE. I really like the phone, but turning into a brick periodically is unacceptable. I hope I get a good one! This phone is *so* much nicer than my old Samsung Intercept!