From various sources on the web, this seems to be an all-too-common problem with the 770 and makes me think it's why the 800 doesn't have a similar sleep mode. My first 770 is now being returned for replacement.

I did find that if it's taken from the 'holster' and power button held down for >10 seconds then the next attempt to power it on works.

My own unit powered off rather than sleeping (at least there was no way to wake it), rebooted occasionally and very seldom would switch itself on from off. Different things happened when externally powered. Shame as it's generally a really cool bit of kit.