This has happened across all saves, and i've tried everything, getting annoyiing now.
it's started when i finished come fly with me.
http://www.youtube.com/watch?v=medUZZA5YVQ

I've had this many times. For me it's a matter of going into 3rd person, holstering my weapon, going back into first person and drawing my weapon. That usually fixes it so long as I'm on even ground.