Default [Q] IMEI NULL/NULL Unknown Baseband

Hi, I got a T-Mobile Galaxy S4 (SGH-M919), after a couple months the phone stopped reading SIM cards, when I checked the IMEI (*#06#) it shows as NULL/NULL and in the settings Baseband Version is unknown.
I tried reflashing my ROM but nothing helped, last week I got a new SIM card reader replacement for my phone, it worked for a day, but the next morning my phone stopped working again.

Any ideas what to do about it would be very much appreciated.
Thank you

I'm currently on KitKat 4.4.2 (the problem started while on 4.2.2).

Update:
When I clear the cache and dalvik cache, fix permissions in CWM (I'm not sure which one helps - it's a bit random), the phone starts to work again and I can make calls, but after about 15-20 minutes the phone loses any signal, when I restart I'm back to Unknown Baseband.

Sometimes I'm able to get the phone working again through EFS Professional - Fix Unknown Baseband, but then again it doesn't last long.