Very bad signal on iPhone 4 after upgrade to IOS 6

Question

After upgrading my iPhone 4 from IOS 5.1.1 to IOS 6, I started having lot of troubles with signal and signal strength: the phone is continuously switching form 3G to GPRS/Edge, signal strength is always bad, Internet access is almost unusable, and sometimes I can't even make phone calls or send text messages; even when it seems to have good 3G signal, as soon as I try to use it to do anything (even a phone call), 7 times out of 10 the signal drops.
This doesn't seem to depend on the places where I go, and anyway I know for sure I'm now having signal troubles in the exact same places where I had perfectly good signal before the IOS upgrade. I'm in Italy, my carrier is TIM, but again, it worked perfectly fine before the IOS upgrade.
Steps I tried to fix the issue:
Settings -> General -> Cellular -> Cellular Data Network -> Reset Settings
Settings -> General -> Reset -> Reset network settings
Using iTunes: backup, reset (also downloading a fresh copy of the IOS image), restore from backup
Nothing seems to have worked.
I've been searching around, and it looks like many people have had similar problems after the IOS upgrade, although some of them seem also to have resolved it using some or all of the above steps; anyway, they were not enough for my phone.
Is there something else I can try? Is this a known IOS bug? Is Apple working on some fix?

Answer 1

Looks like the reset actually solved the issue. I posted the question soon after having done it, and the signal was still bad, but that was probably due to the place I was in. I've tested it for a whole day, and now signal strength is indeed like it was before upgrading to IOS 6.
I also called Apple support, and they confirmed is a known bug, and a full phone reset solves it.