Originally Posted by pnz
I think I've solved this. Used the repair NV data option (to fix broken baseband) in EFS Professional, and it seems to be working fine. Will see if it's still working after a few days though.
My best guess is that this was a file permissions error.
No, not fixed. Worked fine for a few days then the problem came back. Just tried a non-stock ROM (Omega) and same thing. Trying a CM-based ROM now (ParanoidAndroid), but not too hopeful.
Still don't understand why it works for hours/days, then suddenly stops. Rebooting it fixes it the first few times, then that stops working as well (and I get baseband unknown at this point). Flashing a new ROM always fixes it, for a few hours/days anyway.