I have the same issue. I brought my unit to apple service center, and they diagnosed that my unit which was just 1 week old has a hardware issue. Unfortunately I was told that they cannot replace my unit since i bought it from a telco operator. Instead, they replaced everything inside, including batteries. So, probably the only thing new on my unit now is the case and the screen. So disappointed; not going to buy apple again, complete waste of money.
And the worst part is, I barely see any improvement with the signal streght.
I am suspecting, they throtlled down the signal transmit/receive power of the device to compensate for the overheating problem which they claim is resolved in the latest sw update. So, for those who may aleeady have weak signal coverage in their area will definitely notice the drop in signal level on their device.