Try changing the vid_xboxSyncMode in the config file, see below.
vid_xboxSyncMode
Sets how to sync to the vertical blank.
0 = No syncing, game will run as fast as possible, screen may tear
1 = Sync to vblank, locks game to 50 (PAL50) or 60 fps (PAL60 & NTSC)
2 = Sync to every second vblank, locks game to 25 (PAL50) or 30 fps (PAL60 & NTSC)