Iphone 3GS ios 6.1.3 GSM(phone) signal problem????

Recently had some signal problem in my iphone(jailbreaken) so i made i full restore to version 6.1.3 to be certain that there is no incompatibility.
After the restore i keep low signal phenomenon comparing to other phones and this phone comparing with the past.

Firsi of all i notice that if i turn off 3G then all my services(wifi and telephone signal) are out of order with the stady message ''no service''.

I comply with that and let 3G on. In house and other areas where the signal is very strong phone works fine and so the wifi.

In work and other areas with much low signal, due to location, i now get 1 bar and searching and also my wifi disconnect and connect without dropping the signal!!!!!!!!!!!!!!!!!!!!!!!!!

If i move in a really strong signal area again signal with up to 2 bars then the wifi remains good and fuctional!!!!!!!!!!!!!!!!!!!!!!!! (as in my house)

Please help because the coverage in work is needed and was just before 2 days ago just fine!!!!!! I put an other sim card in the phone and still nothing and i test my sim card to other phone in the same area and working fine .

Please help