Hi. My iPhone 4s has same issue.
but, you don't need to get any stress about it.
I went to local apple store, they said unlock iphone 4s with third carrier has same problem of signal issue.
after calling and few second low signal.
I had gotten so much stress of it on realizing the problem issue.
however, I reached godliness about it. -_-
if you won't have used iphone 4s forever, it's better for you to purchase iphone 4s with unlock version.
or waiting for iphone 5.
Well I am having the same problem but worse today on a factory unlocked Iphone 4s in Miami Florida, I have full bars, but then it goes low for a few seconds, but 99.8% of the time it stays with full bar. But the main problem I am having is that I would have full bars, but then no data single at all & I have a circle were the data symbol is & I can't make calls at all or do anything at all that uses data the only thing I can do on the whole phone period is text. But then if I restart sometimes once or some times two times in a row my phone I get the "E" back on my phone & then if I make a call or receive a call & talk for like a min or two no more then that it drops the call & I go back to the circle. For me this has been going on since 2pm Easteren time today (1-3-12) my question is would service be restored & should I worry about it being just my phone?