Thanks for so fast reply!
My Arduino board is powered by a 9V battery and is independent of Motorshield, which is powered by 5xAA batteries. So power is separate. 5xAA batteries I connects to EXT_PWR on Motorshield board and ground must closed on this 5xAA batteries but I don't know what happens. I read that dc motors emit electrical noise, which reboots my Arduino board because it don't have enough power when my motors run. But how I can to change it?
What I must to do?