No service problem iPhone 4

Ive iPhone 4 since december.
But the problem is, my iPhone 4(up-to-date software and no jb) shows no service sometimes.
no carrier&network is found When i go to settings->Carrier settings.
But when i enable 3G the network comes immediately!
Or network comes after reboot!
I tried to reset network settings / all setting many times but the problem continues.
Is this problem familiar to you? Is this a hardware problem?
Any help will be appreciated, thanks!
iOS 4