[U-Boot] Problem with burning Kernel

Hi, I have a strange problem that causes a lot of difficulties to our production . My aim is to burn the Kernel. On some boards in the production there's this problem: If I write to the command line: nand erase 0x400000 0x200000 and then tftp and then write the Kernel to the NAND it works fine.

But if i first do tftp to get the Kernel and then erase the NAND, everything is stuck, and only pulling out the power helps.

Am i doing something wrong? Do you have any idea?

Thanks, Alex