The SIM has nothing to do with the GPS. Run GPS Status & Toolbox and see how many satellites it's locking to (green) and how strong the signals are (the bar graph). The GPS antenna connection could be bad, giving you a few weak signals instead of the 7 or 8 strong ones you should be getting. I'm getting 11 green at the moment. (And 1 grey, which means that the satellite is supposed to be there, but the receiver isn't hearing it.)