I also have noticed some delays or refusals to reboot after shutdown.
I did however discover that hitting the power button again does prompt a quick restart when this happens.
I, too, have noticed that occasionally my 750 seems to lock up at the 'loading maps' screen. I know that this shouldn't be, but I've found that if I shut the unit off and then immediately turn it back on, it goes right to the password screen. Give it a try.