Not receiving network signals on samsung S3 T999?

Hi I just bought a Samsung S3 T-999. The problem is when I switch cellphone on, it connects to my network but just for a second. The moment that second's gone, I lose signals completely. So it's not like imei is corrupted or something. moreover phone is not asking for unlock code at all so i assume it is unlocked too. What could possibly be the problem? How can i solve it?This is the screenshot.