[Q] GT-I9505 Imei no: Null Problem topic

Hi all,
Yesterday my phone was working perfectly but this morning there wasnt any service signal.
I cant call,send messages,,,,it cant read sim-card.
I tried flash stock rom with odin v3.9 , wiping data/caches files. This solved my problem only for 2 hours. After 2-3 hours Imei lost again.
When i dial *#0011*# there is only blank screen.
Is there any solution for Null Imei No ? (I have original Imei no btw)
Should i Root my phone?(No experience with rooting).
What sould i do :confused:.
4.4.2
ap: I9505XXUGNK4
csc: I95050XAGNK4

Thank you.