I've just figured out a great way to play Popcorn movies on the chromecast, without the horrible stuttter associated with tabcasting: Using the great Bubbleupnp android app.
This is what I did :
1. Open the Popcorn app on your PC and play a movie
2. Open the Bubbleupnp android app
3. Select your 'chromecast' device from the 'device' tab
4. In the 'playlist' tab, click 'Add Stream url' in the menu and enter http://192.168.0.5:8888
Obviously replace the IP address with your machine's IP address.
5. It will play just fine and much better than using other chrome tabcasting techniques you may have read about.