Shutter won't close on Nikon D70s

I bought a factory demo D70s from Cameta Camera and sent it back for repair after two days: the shutter opened but did not, and would not, close again. They repaired it and sent it back. It worked for a while but now what's happening is that the shutter will open when I push the shutter release button (the button you push to take a photo) and the shutter opens but does not close again...when I push the shutter release a second time the shutter will close. No photo is recorded on the memory card when this happens. I suspect that the problem MIGHT have something to do with the memory card because the memory card is not approved for use with this camera by Nikon; that is, Nikon does not guarantee that it will work because they didn´t test it. My memory card is a Hitachi 4GB. I bought it at the same time I bought the camera; the sales person didn´t say that it would be a problem and he must know about these cameras. Any help would be appreciated. Thanks!