Flash failures working with 56f8300

We're working in a design with 56f8300 and flash seems to get broken very often: when trying to download (with Codewarrior) we get messages like 'failure erasing flash' in some boards or 'error occurred while programming flash' in others. This error is permanent and starts with only tenths of cycles.

 

We're using the same interface from JTAG to pararell port as evaluation boards and using Codewarrior 6.0 so what are we doing wrong?.

 

Thanks