download the flash software, the only reason it does that is the V1 software they used on it, has issues with the amount of files which were stored on the phone memory, once it got to a certain amount the phone would freeze and reset.
I had mine updated at Carphone Warehouse around 3 months back, word of advice make sure you save all of you details as it will wipe all of the phones history.
I am hopefully getting the 850i next week, well thats if Carphone Warehouse actually get them in. They were meant to be relesed on the 5th, but due to issues with customs this has been set back, or so they say.