You need to wipe the device to downgrade the OS. Therefore, make sure you backup device data and 3rd party apps. Download bbsak to wipe and reload the OS..
- download and install 320 on pc (Remove all other os's from pc)
- delete vendor.xml file
- run bbsak
- wipe the device...this should initiate the OS loading process.
- restore data from backup.
Posted from my CrackBerry at wapforums.crackberry.com