Split screen signing in issue with gears of war 2

Ok, so when I'm on the main menu and my girlfriend presses Y and logs into her xbox live account which she has just renewed, the game freezes and I have to reboot the xbox. However when she presses Y and logs in as a guest, there is no issue and it works fine.

I have tried everything but everytime she trys to use her gamertag the whole xbox just jams up?

Any advice on this would be appreciated.

i haven't tried that i'll have a go in an a bit

Ok, so i signed out of my profile on the dashboard and signed into my girlfriends account. I then loaded gears 2 and it wouldn't even go to the main screen. The screen turns black and the xbox jams. To get it to work I have to re-boot the xbox by turning the power off, which then goes back to my profile.

To be honest this has only started to happen since I downloaded the update for the dash a couple of days ago. Should I contact xbox support. or has anyone got any ideas.