How did you revert back to a previous OS? My GPS also wont lock, and i'd rather have the other OS back than this one.
The same way you would when upgrading the OS: Download the OS version you want, run the installer, connect your BB to DM and follow the prompts. It asks are you sure you want to downgrade to an earlier version.
Mine works on 4.5 with AT&T, but is VERY slow to lock on, every time. 4-5 minutes is standard.
If I downgrade to 4.2, it locks on in 30 seconds or so. However I then lose the advantages of 4.5