I, too, have this plague (with my N800)!
I reflashed to Diablo, and after undoing Lifeguard, it booted after several attempts. Before the no LG, the bar would go across a third of the way, then reappear near the far end (weird) and hang!

I had to reboot after a couple of days due to the browser not launching due to "insufficient memory"(!)
Now it won't boot at all; typically the blue bar makes it across the bottom screen all the way, then the loop recurs after a total of 45 seconds.

I will reflash to 2007,(and reset no LG) but I still did experience looping under it as well (just not as persistently).

At this point I'm wondering if it's a h/w problem. I'm tempted to re-watch that video of an N800 being dismantled, and just going in and re-heating soldering joints in case there's a disturbed/cold one somewhere (I'm only half-joking here). It's out of warranty now, anyway. I wonder if there's such a thing as a "lower level" re-flashing (akin to earlier hard drives).

This device is providing no end of frustration! At the same time, I do like it when it's working!