I wonder if it could be based on where in memory the hex file is being uploaded to. Maybe there is a problem with certain ranges. Also, are the hex files that work produced by the same tool as those that do not? There are at least two hex formats - Intel Hex (.ihx) and regular hex (.hex) and they are different. I've seen some programmers that work with small programs in .ihx format but fail for larger programs. The solution is to use .hex format in this case.

Frank Kienast