iPhone 4 network problem

My iPhone 4 loses signals all of a sudden and it won't detect the network again( it just says searching and shows 'no service' even if i go to an area with full network), switching on airplane mode nd switching it off doesn't make any difference, i tried resetting the network settings which also didn't make any difference, the only solution is i have to switch off the phone and switch it on in order to make the phone get back its network. It happens everyday and its really ******* off to restart the phone several times per day. I seriously can't trace out the problem, i replaced the simcard too.

i am using the phone from 4 months and i have been facing this problem for the day 1!