I had the same problem, and really don't have a fix for you. Here's how my story went. It took a while for me to download G. Hornet and the Spider-Man demo. Once I did, G Hornet worked fine in 3D, and so did Spider-Man. Didn't watch the whole move, and shut it off because I wanted to watch it on my 3D TV, which I cannot connect to...I'll save you the story about that one. A day later when I found out I could not get the movie to work on my 3D TV, I decided to watch the movie through my phone again. To my surprise it would not play in 3D! I tried it at least 3 times and it still would not work in 3D.
This is what I did:
1. Shut the phone down and let it set for at least 15 mins (might want to take the battery out also, and put it back in). (
make sure your phone is off before taking the battery out.)
2. Didn't try to watch G Hornet for 2 days.
3. Made sure I had a great WiFi connection.
(I can barley get a G4 connection and I'm right outside of the St. Louis area.)
G Hornet works fine now, just tested it out around 5hrs. ago. I did have a issue with You Tube tonight. Some of the 3D movies did the same thing. It would work in 3D, then when I loaded a new 3D You Tube vid it would quit working. Pretty much all I did was reload the movies till it worked in 3D. The weird thing was when I backed out to the You Tube, the menu on the top part of my screen was still in 3D with double imaging, then the vids I chose from from were in 2D. I could totally see a double image at the top of my screen where the You Tube logo and search bar, but the 3D vids that were listed didn't work.
Just let us all know if this fixes your problem with the restart.