There have been a number of posts detailing loose crystals. However, it seems that the crystals are soldered in, so it isn't just a case of just pushing the top back in. Some people do that, and add some tape to keep it in place.

A few guys have described how the circuit board had tiny cracks, and that allowed the crystals to work loose. Personally, I'd take it back and get another one. It is a fine electric trainer plane, but does have some issues with the radio system. I've switched mine over to a DX6 system. Rich