Have you read through
this thread? I was having the exact same problem early on. I replaced the phone and when that didn't work, I had my Nexus boxed up and ready to exchange for a Rezound.
Instead, based on that thread, I tried swapping out my SIM card and it seems to have fixed my problems -- enough that I plan to keep the Nexus past the Friday return deadline. Per the thread, the SIM swap is hit or miss for others so I have no way of knowing what the end all solution is, but I figure that should it recur in the future, that has got to be something that falls under the 1-year warranty.
I do NOT think custom ROMs will address this issue.