Maybe this will help someone, or bring us closer to solution.
Hey, I have similar problem. Iphone 4 looses carrier randomly, normaly after few hours. If I just reset it, it doesn't help.
However, what helps is this: I wrap it in a plastic bag or plastic foil for food and put it in a deep freezer for a hour or so.
Then I take it out of the freezer, wait few minutes that it warms up a little (this is the critical part for moisture ---thats why it is in a foil) and then power up and it works for another few hours.