i just replaced my 3gs and i have a new factory unlocked phone same problem keeps happening to me too
i think it has to do with apples last update 5.1
i hade the problem with an unofficial unlocked phone but now the phone is a brand new replacement from apple so i don"t think it is your phone i think it has to do with an update or network problem.
also i notes when when i use internet than the phone drops the signal.