Same problem to me. I have a new S4 from 2 days, bought "refurbished" from aliexpress at a good price. It's new, looks great but signal and in call audio quality is weak.
Phone works good on "GSM only". If i set GSM/WCDMA, I lose signal and phone stops itself.
I started with stock 4.0.3, phone stops itself.
Updated to 4.0.4, phone doesn't stop but no signal with WCDMA enabled.
Then I unkocked, rooted and tried Miui V5 (4.1), same problems.
After installing Maximus HD, I had signal also with GSM/WCDMA, but immediately I installed newer firmware and signal was lost. (Works only on "GSM only").
Tried also Beanstalk, CM11, MagioRom, SourcE, Slim kat. No signal with GSM/WCDMA.
I don't know if works with Maximus with older firmware, or it was only a coincidence. Don't know what firmware was at that moment.