Hi,
What exact IOS image are you trying to use? Can you also post the output of the verify /md5 after it is downloaded to the FLASH? Can you also try using HTTP to download the file into the switch? Last but not least, how about dir flash: - does the downloaded file size match the size of the original file?
If this all works out - I know this might be a very stupid suggestion but - can you actually afford to boot one of your switches with the new IOS, and if it boots up properly, verify the MD5 sum afterwards? There is a slight possibility that the verify /md5 command isn't working entirely right in the old IOS you have. You could always keep your current IOS backed up so if the new IOS does not boot, you will revert to the old one using XMODEM. This would understandably cause network downtime, however - restoring an IOS image over XMODEM on 115200 bps will take tens of minutes.
Best regards,
Peter