Default [Su640] Weak 2g signal on Stock rom

Hi there,

I am facing an issue related to 2g signal on Su640 stock rom v30d.
2g signal is almost 0, and sometimes the phone shows emergency call only. I must switch network mode to 3g in order to make calls.

If I flash CM10.1 rom, 2g signal is very strong.

Do you have any idea how to fix this?

Thanks in advanced!