I have the sport graphite too and am very pleased with it. If your satnav fails to lock on or loses the satellites, you need to get the dealer to move the GPS antenna from on top of the unit to higher up under the dash, they moved mine to under the speedometer curved cover. Once that was done, my satnav has been flawless. Also, it is possible the maps haven’t been updated (which is expensive!!).
Have a look at a couple of my other posts for the satnav.