We are using custom bootloader to write main program to flash. This has been working fine over the 150 boards, but now we got one board that fails in the flash erase phase. The reason to this fail is that flash erase is taking more time than before. The bootloader erases flash as much is needed for the new code. It looks that abnormal time is taken in the last erase phase where is used sector erase. The main program flashing seems to work fine if time out is increased. Also the board is working fine after that. With the normal boards erase has been taken about 2s, but in this one board it takes about 0.5s more.
Any ideas what could cause this? I have been thinking could the problem be in the board HW? We are also unsure is that that board usable or should we try to replace MCU on the board.