canon10d and 580exII flash

I have the 10d and 580 and when I put the camera on shutter priority i get a wide vaiety of exposures even when nothing changes.. same subjuect same position and distance

anyone got some ideas

Light coverage will be varied by aperture AND shutter, not just one or the other. You'll want full manual for real control, you are asking for trouble putting a flash on in an automated mode.

Also, pay attention to your bounce direction, a slight 10 degree angle change can make the difference between good and bad light coverage.