I have faced similar type of issues and resolved after updating the software by 3S. They never said such things that usb is heavy or contains more songs. The guy there spent time investigating and recommended software update.
One issue is still there - that's weak radio reception and its in all cars - I have observed that in few other grandes as well. However, they said that if i want they can log complaint and will replace the head unit in warranty. I didn't go for it as this replacement would be just for nothing.