It it's still RSODing in service mode, then it's registry may be corrupt (The only thing that doesn't change with a downgrade) Downgrade it to 3.41 (Or another working firmware) jailbreak it, and use a file manager or FTP server to:
1) Make a full backup copy of dev_flash0, dev_flash1, and dev_flash2 on your computer.
2) Copy a file called "xRegistry.sys" from "dev_flash2/ect/backup" to "dev_flash2/ect" (Overwrite the existing one)
This should give you console a clean registry to boot from. Now, reboot the console, and upgrade back to 3.55.