[U-Boot] Continuos reboot in u-boot 1.3.4

Hi

I am using u-boot 1.3.4 ( got from atmel) on AT91SAM9G20 , If enable the ethernet related code in the u-boot ( drivers etc) the board starts resetting again and again continuously without giving any error message. Please somebody tell me what type of mistakes ( coding errors ) make the board to reset in u-boot. Or it is because of writing some wrong values in the controller resisters. Thanks