Bought my S3 from T-Mobile a few years ago, have installed a number of roms on it, but most recently updated to CM 10.2. I started having boot issues (phone would stop booting after a couple of restarts, not a sudden death syndrome phone, BTW). After using ODIN to load on root66-UVUEMJC (tmobile 4.3), and running into more issues, I ended up loading Stock 4.3 from Kies (yes, I realize this is a horrible mistake now).
That got me on my feet again, I used ODIN to install clockworkmod rom manager, then re-installed CM10.2. It was at this point where I noticed that I no longer had cellular network connectivity anymore. I've tried another t-mobile SIM in my phone to make sure it wasn't that, and it had the same problems:
On startup, my phone will connect to the cellular network and fill in all the fields under Status (including IMEI), but after a minute or so, I loose that and the device goes to "No Service". Nothing I've tried has been able to get this fixed. I have attempted to flash the latest modem as well with no luck.
As mentioned, any help would be greatly appreciated on what to look for / do to get this thing working so I don't have to drop a load on a new phone.