if you had this issue from the start then my first guess would be a incompatible sim-card/network. not all gsm networks use the same frequency(s) and i've seen certain type of sim just not work at all. AFAIK these devices are not network loacked, although a few people have requested a unlock solution I dont belive they are loacked and it's more simcard/frequncy problems and/or user error.
there is a hidden menu where you can switch network type
in the stock dialer type
pin is 0821
from there you can check your settings.
another thing ...after u said it may be due to flashing ,i asked my brother and he said that signal problem occur after flashing a rom...so ur guess is right that occur after flashing....so this problem can reslove????