This whole fiasco was a great learning for me. Lost imei and baseband on my way and recovered them too. And looks like it will end in me performing a DIY on my phone.
For now the signals are stable for more than 12 hours which is like the most I have seen since 3 days. I will see if I can utilize your guide this weekend.
Thanks again for your help and time.
Can loosing imei/baseband be the cause of loosing connectivity?
EDIT 1: And my troubles are far from over. It still needs to be switched to aeroplane mode and switch it off to get the connectivity back or a reboot is required. The worst problem is that now the signal shows full in the status bar but with "No connectivity" or "Emergency calls only" which is even worse to handle.
Looks like it can connect to the network provider normally but cant hold the connection for long and requires a reset. Will changing USB port help in this kind of case?
EDIT 2: After a whole day of being on stock and switching the phone into aeroplane mode again and again, I thought of giving custom roms another try. On a KK ROM and on another JB ROM (4.1.2) it was a total struggle to even connect to a network. Tried various modes - wcdma, gsm, auto but nothing worked. Tried different modems too but it just ended up in making the phone really hot. Experimented so much that even when the sim was in, i got the message - "insert sim card to access network". Back to stock rom now.