I have problems with version 2.0 of the gprs shield, I think the 5v of USB Arduino that fuel is not sufficient to feed even the modem 
… with version 1.0 I’ve never had problems but now with the new modem powered by arduino too many strange things happen.
Plus, if I feed arduino with only the USB drive into your computer’s modem goes off … 
how come you chose to power through arduino if this does not make it?? 
bye