Iphone 3gs Ios6 1 Bar signal

Hi, i have a 8gb iphone 3gs, baseband 5.13.04, ever sinse i updated to ios6 i have had really bad signal mostly 1 bar it flickers from no service up to full bars, but because of it changing constantly its unusable, also if i had good signal and i turned on 3g, the phone automatically has no service, it is locked to 02

please helps!!