Did you just root or install a custom ROM. Root isn't supposed to affect the ability to update PRL. I would seriously consider a odin back to stock and see if that fixes your problem. My recommendation would be to not update the PRL, its the only thing I can think of (and others have noticed too on XDA) that has given me piss poor 4G performance. GF hasn't updated hers and gets faster connection speeds on 4G (bone stock, too, will not let me root her phone).