I've had this occur once or twice..but other T-Mobile USA users in the same area were also experiencing problems (with other devices), so I wrote it off as network issues.
There are some apparent issues for the Galaxy Nexus when handing off between WiFi and cellular which should be addressed in 4.0.4+ if/when released.
The only other thing is that there are some users reporting better luck using different Baseband versions on this device.
Can't post much here due to terms, but assuming your GSM Galaxy Nexus actually has a valid warranty..this will likely cause grief. Anyhow, baseband discussions and recommendations have come up many times over @ XDA:
Hope this helps.