Hi all - I'm looking for a bit of GPS-related help please...

I have a Galaxy S4 i9505 that's been working fine for the last year, on a custom 4.4.2 custom rom (I forget the modem baseband), until a few weeks ago when the GPS stopped locking on to any satellites.  I then decided it might be a good time to check out an Android 5.0 rom and hopefully restore the GPS functionality.

No luck.  If using Google Maps for navigation in the car, it can take some 15 mins before it tentatively locks on to a satellite.  But it's next to useless and easily loses the lock.  I'm now wondering if it's an issue with my baseband not matching the bootloader, or CFC code?  (at this point it begins to go over my head!)

My phone's details:

Bootloader version:  I9505XXUDMI1
Baseband version:  I9505XXUGNK4
PDA Version:  I9505GUEUDNL3
CSC Version:  I9505GXARDNL3 

Original CSC Code:  XAR
Firmware CSC Code:  XAR
Active CSC Code:  XAR
CSC Country:  USA
CSC ISO:  US
Product Code:  GT-I9505ZWAXAR

Does this make any sense to anyone?  Should any of these codes be matching in order for the GPS to function correctly?  

Thanks in advance!
Chris.