Default [Q] To keep connection I have to switch between GSM Only and GSM Auto (PRL). Why?

So, I've been using the LS980 on MetroPCS fine using Justin's ROM, then the CM11 T-Mobile made for this phone - for a while at that. A poster commented that will the Telinfo app, I could use any ROM. Trying, I never got a signal. If I did, it would last for a few hours.

I went back to stock and tried again - using the LS980 CM11 ROM for Sprint, I've been able to keep a connection the longest. Using the Telinfo app, I'll be on GSM Only, then it would go to 'E' or 'Emergency Calls Only', so then I switch to GSM Auto (PRL), and I'll be back to normal speeds. Usually once to twice a day I have to do this.

Any idea why? Did I somehow screw up the radio? As I said, I went back to stock and it picked up the Sprint network fine (though I couldn't use it with no Sprint service).